Based on other forum posts it sounds like support for .mp3 files has been unofficially dropped, and you should use .ogg files instead. Honestly I'm not sure why anyone is still using .mp3 files in addons, as it was pretty strongly implied years ago when they introduced .ogg support and switched all the game's own sounds over to .ogg that they were going to drop .mp3 at some point. Monkey see, monkey do, I guess.
