[Sean Parker]

So… I know this thread is over a year old, but as it is currently the first Google result for “Premiere GarageBand glitch” I thought I’d throw in the solution that worked for me in case anyone else is stumped.

It all came down to Premiere and GarageBand being open simultaneously. My system didn’t like that. After quitting BOTH applications and then reopening only Premiere, the sound returned perfectly to normal again.