It's using the midi plug-in to play midis in the browser now instead of opening media player. Their website is no help (they have no email support 😱 )
Annoying problem with QuickTime
Did you try to reinstalling it? I dunno what else to tell ya
Ive had my share of problems with quick time...I used to be able to fastseek backwards with audio intact, now it wont do that anymore. And yes their lack of support is a PITA
Actually I solved the problem. I had been thinking since the problem was with Quicktime that I should fix it in Quicktime. But then someone told me to reinstall WMP so it would fix all the file associations. I didn't have to reinstall it, I just reassociated midi files in options.