Sound scratchy

Seems to me that the first thing you want to do is find out if your problem comes from the camera speakers/output, or if it records them scratchy. Take one of the "scratchy" cassettes, and play them on another device to see if it transfers bad audio. If it does, then you're worst off offcourse, more of a repair. If it doesn't you can decide you don't need a repair, as long as it does the job.

Good luck