Ran into this issue too. This symptom has been reported already with an issue (firefoxpwa: Error no decoder found for audio/mp4a-latm · Issue #505426 · NixOS/nixpkgs · GitHub). The proposed solution still works with the latest nixos-unstable commit.
I looked into firefox/firefox-unwrapped/wrapper but today is my first time working with firefoxpwa so I’ve not gathered enough understanding yet for a proper fix.