I created the bug QTBUG-98373 which partially fixed the issue with allowing mp3 files to be played on wasm.
There are only two issues left on desktop/Linux Firefox keeps asking for webcam permission and it even turn it on, it seems the permission code asks for that, and second and most important to me is that on mobile devices the audio output seems not to be set, from my tests there is support there but the Qt code fails to select the proper output.