README: Mark mute sounds as finished

Author: adazem009

README.md

@@ -110,7 +110,7 @@ When the project loads, click on the green flag button to run it.
 - [x] FPS options
 - [x] Stage size options
 - [x] HQ pen
-- [ ] Mute sounds
+- [x] Mute sounds
 - [ ] Theme options (light/dark mode, accent color, etc.)
 
 See the [open issues](https://github.com/scratchcpp/scratchcpp-player/issues) for a full list of proposed features (and known issues).