Can't set default music app

I can't have the app 'music' as my default mp3 playing app. When I click on a music file it now always plays using the 'Multimedia UI Service Layer' app. I've googled and tried everything except a factory reset (which I'd rather not do cos I don't want to lose my open Chrome tabs). I've cleared all the default applications, I've cleared the caches of all the media players. Any ideas?