@@ -5,7 +5,7 @@
 use crate::ffi::*;
 
 use crate::regions::SpeedSegment;
-use crate::scene::SceneAudio;
+use crate::scene::{SceneAudio, SceneAudioTrack};
 use anyhow::{bail, Result};
 use std::f32::consts::PI;
 use std::ffi::CString;
@@ -1381,6 +1381,84 @@ pub fn assemble_concatenated_pcm(
     output
 }
 
+/// Mix imported audio tracks (issue #350) over the assembled programme.
+///
+/// Each track is decoded across its trim window — already resampled to 48 kHz
+/// stereo by `decode_clip_audio`, the same path a clip's own audio takes — scaled
+/// by its per-track gain (the same `10^(dB/20)` law as `finish_audio`), and summed
+/// into the programme at `start_sec`. The programme length is NOT extended: a
+/// track that runs past the video is truncated to it, so the audio and video
+/// streams stay the same length for the muxer.
+///
+/// The decode window is capped up front at the room left in the programme after
+/// `start_sec`, and a track starting at/after the end is skipped without decoding.
+/// `decode_clip_audio` preallocates from the window, so this keeps a long track
+/// pinned near a short programme's end from buffering (and clamping away) hours of
+/// PCM. `trim_end_sec` must therefore be concrete — the renderer sends
+/// `trimEnd ?? durationSec`.
+///
+/// A track whose file has no decodable audio is skipped — the same degradation a
+/// stream-less clip gets.
+pub fn mix_external_tracks(mut programme: PlanarPcm, tracks: &[SceneAudioTrack]) -> PlanarPcm {
+    let programme_len = programme.first().map(Vec::len).unwrap_or(0);
+    if programme_len == 0 {
+        return programme;
+    }
+    for track in tracks {
+        let offset = (track.start_sec.max(0.0) * AUDIO_OUTPUT_SAMPLE_RATE as f64).round() as usize;
+        // A track that starts at or past the programme end contributes nothing —
+        // skip it before decoding anything.
+        if offset >= programme_len {
+            continue;
+        }
+        let trim_start = track.trim_start_sec.max(0.0);
+        let Some(trim_end_full) = track.trim_end_sec else {
+            // Without a concrete end there is no safe window to decode (see the doc
+            // comment); the renderer always resolves one, so this only guards a
+            // hand-written scene.
+            continue;
+        };
+        // Cap the decode window at the room left in the programme. Everything past
+        // `offset` that overflows is discarded by `overlay_track_pcm` anyway, so
+        // decoding it only wastes time and memory — a three-hour track placed at
+        // second 9 of a ten-second export must not buffer three hours of PCM.
+        let remaining_sec = (programme_len - offset) as f64 / AUDIO_OUTPUT_SAMPLE_RATE as f64;
+        let trim_end = trim_end_full.min(trim_start + remaining_sec);
+        if trim_end <= trim_start {
+            continue;
+        }
+        let decoded = match decode_clip_audio(&track.path, trim_start, trim_end) {
+            Ok(Some(pcm)) => pcm,
+            _ => continue,
+        };
+        let gain = 10.0f32.powf(track.gain_db.clamp(-12.0, 12.0) / 20.0);
+        overlay_track_pcm(&mut programme, &decoded, offset, gain);
+    }
+    programme
+}
+
+/// Sum one decoded track into the programme at `offset` samples, scaled by `gain`,
+/// truncated at the programme's end. Split out of `mix_external_tracks` so the
+/// placement/gain/clamp math is testable without ffmpeg, exactly like
+/// `mix_aligned_tracks` is split from the decode above.
+fn overlay_track_pcm(programme: &mut PlanarPcm, decoded: &PlanarPcm, offset: usize, gain: f32) {
+    let programme_len = programme.first().map(Vec::len).unwrap_or(0);
+    if offset >= programme_len {
+        return;
+    }
+    let room = programme_len - offset;
+    for channel in 0..AUDIO_OUTPUT_CHANNELS {
+        let Some(source) = decoded.get(channel) else {
+            continue;
+        };
+        let count = source.len().min(room);
+        let dst = &mut programme[channel];
+        for k in 0..count {
+            dst[offset + k] += source[k] * gain;
+        }
+    }
+}
+
 /// Encodeur AAC attaché au muxer avant son header. Les paquets utilisent le même interleaver
 /// que la vidéo ; les pts restent en unités échantillon jusqu'au rescale vers l'AVStream.
 pub(crate) struct AacEncoder {
@@ -1692,6 +1770,64 @@ mod tests {
         assert_eq!(mixed[1], vec![0.25, -0.5, 0.75]);
     }
 
+    // Imported audio track overlay (issue #350).
+    #[test]
+    fn overlay_sums_at_offset_with_gain() {
+        let mut programme = planar(&[0.1, 0.1, 0.1, 0.1]);
+        // ×2 gain, placed at sample offset 1.
+        overlay_track_pcm(&mut programme, &planar(&[0.2, 0.2]), 1, 2.0);
+        assert_eq!(programme[0], vec![0.1, 0.5, 0.5, 0.1]);
+        assert_eq!(programme[1], vec![0.1, 0.5, 0.5, 0.1]);
+    }
+
+    #[test]
+    fn overlay_truncates_a_track_that_runs_past_the_programme() {
+        let mut programme = planar(&[0.0, 0.0, 0.0]);
+        // A 4-sample track placed at offset 2 has room for only 1 sample.
+        overlay_track_pcm(&mut programme, &planar(&[1.0, 1.0, 1.0, 1.0]), 2, 1.0);
+        assert_eq!(programme[0], vec![0.0, 0.0, 1.0]);
+    }
+
+    #[test]
+    fn overlay_past_the_end_is_a_no_op() {
+        let mut programme = planar(&[0.3, 0.3]);
+        overlay_track_pcm(&mut programme, &planar(&[1.0]), 5, 1.0);
+        assert_eq!(programme[0], vec![0.3, 0.3]);
+    }
+
+    #[test]
+    fn mix_external_tracks_skips_empty_windows() {
+        let programme = planar(&[0.4, 0.4]);
+        let tracks = vec![SceneAudioTrack {
+            path: "/nope.mp3".into(),
+            start_sec: 0.0,
+            gain_db: 0.0,
+            trim_start_sec: 2.0,
+            trim_end_sec: Some(1.0), // end <= start: empty window, never decoded
+        }];
+        // The empty window is skipped before any decode, so the programme is
+        // untouched even though the path does not exist.
+        let out = mix_external_tracks(programme, &tracks);
+        assert_eq!(out[0], vec![0.4, 0.4]);
+    }
+
+    #[test]
+    fn mix_external_tracks_skips_a_track_that_starts_past_the_programme() {
+        // 2 samples = ~0.00004 s of programme at 48 kHz; the track starts at 1 s, so
+        // its offset is past the end. It must be skipped before any decode is
+        // attempted (the path does not exist), never buffering its window.
+        let programme = planar(&[0.4, 0.4]);
+        let tracks = vec![SceneAudioTrack {
+            path: "/nope.mp3".into(),
+            start_sec: 1.0,
+            gain_db: 0.0,
+            trim_start_sec: 0.0,
+            trim_end_sec: Some(3600.0),
+        }];
+        let out = mix_external_tracks(programme, &tracks);
+        assert_eq!(out[0], vec![0.4, 0.4]);
+    }
+
     #[test]
     fn single_track_is_not_clamped() {
         // Promesse de non-régression : une source mono-piste ressort telle quelle, y compris
 
@@ -21,7 +21,7 @@ use std::ffi::CString;
 use std::ptr;
 
 use crate::audio::{
-    assemble_concatenated_pcm, build_audio_concat_plan, finish_audio,
+    assemble_concatenated_pcm, build_audio_concat_plan, finish_audio, mix_external_tracks,
     AacEncoder, PlanarPcm,
 };
 use crate::audio_jobs::{decode_and_stretch_clip_audio, ClipAudioJobs};
@@ -459,6 +459,12 @@ pub fn run_composited_multi(
 
     let scene = comp.scene_snapshot();
     let audio_settings = scene.as_ref().map(|scene| scene.audio).unwrap_or_default();
+    // Imported audio tracks (issue #350), cloned out of the borrowed scene so the
+    // mix step below owns them. Empty for a project with no imported audio.
+    let audio_tracks = scene
+        .as_ref()
+        .map(|scene| scene.audio_tracks.clone())
+        .unwrap_or_default();
     // Ring de staging a 2 : l'export ne veut que du debit, une frame de latence
     // ne se voit pas dans un fichier. Voir `Compositor::set_readback_depth` pour
     // la raison pour laquelle la preview, elle, reste a 1.
@@ -552,7 +558,10 @@ pub fn run_composited_multi(
         let declared_audio: Vec<bool> = clips.iter().map(|c| c.has_audio).collect();
         let plan = build_audio_concat_plan(&clip_frame_counts, &declared_audio, out_fps as f64);
         audio_encoder.encode(
-            &finish_audio(assemble_concatenated_pcm(&clip_pcm, &plan), audio_settings),
+            &finish_audio(
+                mix_external_tracks(assemble_concatenated_pcm(&clip_pcm, &plan), &audio_tracks),
+                audio_settings,
+            ),
             octx,
         )?;
         crate::ffi::averr(crate::ffi::av_write_trailer(octx), "write_trailer")?;
 
@@ -30,7 +30,7 @@
 //! décodeurs, symétrique.
 
 use crate::audio::{
-    assemble_concatenated_pcm, build_audio_concat_plan, finish_audio,
+    assemble_concatenated_pcm, build_audio_concat_plan, finish_audio, mix_external_tracks,
     AacEncoder, PlanarPcm,
 };
 use crate::audio_jobs::{decode_and_stretch_clip_audio, ClipAudioJobs};
@@ -1078,6 +1078,11 @@ pub fn run_composited_multi(
     // raconte avoir déjà coûté une fois.
     let scene = comp.scene_snapshot();
     let audio_settings = scene.as_ref().map(|scene| scene.audio).unwrap_or_default();
+    // Imported audio tracks (issue #350), cloned out of the borrowed scene.
+    let audio_tracks = scene
+        .as_ref()
+        .map(|scene| scene.audio_tracks.clone())
+        .unwrap_or_default();
     frames = unsafe {
         crate::timeline_walk::walk_composited_timeline(
             clips,
@@ -1146,7 +1151,10 @@ pub fn run_composited_multi(
         let declared_audio: Vec<bool> = clips.iter().map(|clip| clip.has_audio).collect();
         let plan = build_audio_concat_plan(&clip_frame_counts, &declared_audio, out_fps as f64);
         audio_encoder.encode(
-            &finish_audio(assemble_concatenated_pcm(&clip_pcm, &plan), audio_settings),
+            &finish_audio(
+                mix_external_tracks(assemble_concatenated_pcm(&clip_pcm, &plan), &audio_tracks),
+                audio_settings,
+            ),
             octx,
         )?;
 
 
@@ -3,7 +3,7 @@
 //! tout le run, deux lectures seulement. Rien dans la boucle ne peut fausser le fps.
 
 use crate::audio::{
-    assemble_concatenated_pcm, build_audio_concat_plan, finish_audio,
+    assemble_concatenated_pcm, build_audio_concat_plan, finish_audio, mix_external_tracks,
     AacEncoder, PlanarPcm,
 };
 use crate::audio_jobs::{decode_and_stretch_clip_audio, ClipAudioJobs};
@@ -1344,6 +1344,11 @@ unsafe fn run_multi_inner(
     // fenêtrage par clip ; `walk_composited_timeline` s'en charge.
     let scene = comp.scene_snapshot();
     let audio_settings = scene.as_ref().map(|scene| scene.audio).unwrap_or_default();
+    // Imported audio tracks (issue #350), cloned out of the borrowed scene.
+    let audio_tracks = scene
+        .as_ref()
+        .map(|scene| scene.audio_tracks.clone())
+        .unwrap_or_default();
 
     // ---- encodeur (choisi à l'exécution, cf. ExportCodec::candidates) + mux ----
     // Backend CPU : pas de pool D3D11 du tout. `av_hwdevice_ctx_init(D3D11VA)` échoue sur
@@ -1477,7 +1482,7 @@ unsafe fn run_multi_inner(
         out_fps as f64,
     );
     let assembled_audio = finish_audio(
-        assemble_concatenated_pcm(&clip_pcm, &audio_plan),
+        mix_external_tracks(assemble_concatenated_pcm(&clip_pcm, &audio_plan), &audio_tracks),
         audio_settings,
     );
     audio_encoder.encode(&assembled_audio, octx)?;
 
@@ -432,6 +432,29 @@ pub struct SceneAudio {
     pub gain_db: f32,
 }
 
+/// One imported audio track (issue #350) mixed over the assembled programme —
+/// voiceover / BGM / SFX. Deliberately a SEPARATE `Scene` field rather than a
+/// member of `SceneAudio`, so `SceneAudio` stays `Copy` and the pipelines keep
+/// copying it out of a borrow unchanged.
+///
+/// `start_sec` is the track's head on the OUTPUT programme; `trim_start_sec` /
+/// `trim_end_sec` window the source file (both source seconds). The renderer
+/// resolves `start_sec` from the track's raw timeline position — equal to it when
+/// the project has no trims/speed, which is the case this first cut mixes exactly.
+#[derive(Debug, Clone, Default, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SceneAudioTrack {
+    pub path: String,
+    #[serde(default)]
+    pub start_sec: f64,
+    #[serde(default)]
+    pub gain_db: f32,
+    #[serde(default)]
+    pub trim_start_sec: f64,
+    #[serde(default)]
+    pub trim_end_sec: Option<f64>,
+}
+
 #[derive(Debug, Clone, Copy, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct SceneOutput {
@@ -500,6 +523,10 @@ pub struct Scene {
     /// Global audio finishing. Default keeps old scene payloads bit-for-bit compatible.
     #[serde(default)]
     pub audio: SceneAudio,
+    /// Imported audio tracks mixed over the programme (issue #350). `#[serde(default)]`:
+    /// absent from every scene written before this, and from a project with none.
+    #[serde(default)]
+    pub audio_tracks: Vec<SceneAudioTrack>,
     /// Crop écran par clip, dans le même ordre que `clips` (`cropByClip` côté TS).
     #[serde(default)]
     pub crop_by_clip: Vec<Option<SceneCrop>>,