It has started to work again for some videos, but videos with music in them still do not work.
What version ASB are you running?
Avast Team Member BWhit says in Reply# 1 that this issue will be fixed in the next version. Latest version is now 75.0.1447.81, old one you likely reported issue on was 74.0.1376.132. Version numbers do make a difference as security fixes and updates are bundled in only on newer versions.
Just ran a music video and got sound, was able to download via Video Downloader, and run the mp4 with sound and am running 74.0.1376.132.
It's possible you've accumulated a bit of internet dross in your ASB browser over time.
Suggest doing a Ctrl+Shift+Del and ticking off what you think will clear your issue(s). Just be aware if you go too deep, you can lose your desktop shortcuts temporarily, so if you want to keep them, do a little cleaning at a time until your issue clears. You might be surprised at how much data is stored inside ASB if you've never done this before.
Anyway, your music block could be put down to copyright restrictions, and, depending on what these are, may not be much Avast could, or should, do about them.