Путин хуйло 03/16/2024 (Sat) 00:37 Id: 7fd47f No.456367 del
https://bbs.archlinux.org/viewtopic.php?id=293110

It was "media.ffvpx.enabled" which I had set to disabled. This was recommended for video hardware decoding for a long time but now videos still seem to be GPU decoded when this is enabled (4K 60 fps on YT at around ~3% CPU), and mp3s are working again.
changed media.ffvpx.enabled to true (default value) and this fixed the issue.