Like Mark Raudonis (above) I have experienced this in the past. I would have a music cue in the timeline doing fine and then suddenly it changes into Brass Band music.
This is called “The Brass Band Effect” and is a known issue.
You have not given Final Cut the audio in a format with which it is entirely happy – maybe MPEG3 or something. You may also be hearing little pops and cracks – yes?
In any event you would do best to turn your audio files into Quicktime files which FCP likes. Give us a scream if you are unsure of how to do that.
Best
Harry.