One idea that popped up in my head would be to (and DO export those registry branches first!
) check the HKCR\.mp3 (etc), check out the "default" key (on my system that'd have the value "mp3file") - then nuke the .mp3 and key (and all subkeys of course), plus the key mentioned by the default entry. At least that'll get rid of the keys, including permissions - hopefully whatever player you want will be able to fully construct what it needs afterwards.
The trick would work for me, but I don't depend on any explorer.exe support for media types etc., I just want to double-click my media files and have them enqueue in foobar...
Anyway, once you have things working, you can set permissions for those registry keys and only allow read access - that might
work for getting media player to ACCEPT not stealing ownership of the permissions