Fix audio source tags

[aatanasov-cloudinary]

Fixes audio source tags.

Approach

• Ensuring audio source tags aren't converted.

QA notes

• Go to a post and insert an audio file.
• The audio file should work correctly on the front-end.
• Install the classic editor.
• Go to a new page and insert an audio file.
• The audio file should work correctly on the front-end.
• There shouldn't be any errors.

[gabrielcld2]

Works as expected ✅