Audio Quality when Editing Video

Hello!

In my currenty project I have 2 video tracks and one separate audio track that I imported from garageband (via itunes). Right now, I'm trying to switch between the two video files, by spliting the video in the right chunks, and changing the opacity so only one or the other is showing (hopefully this makes sense).

When I split the video file or edit it, the audio file gets very fuzzy. In fact there's this weird sound in the background, that almost sounds like a record spinning around or something like that.

It's not in the itunes/garageband file, so iMovie is adding it or making it somehow.

Has anyone else experienced this?? And is there a solution?

Thank you!

-Nick