Are you saying that the stray audio is coming from continuous clips which suddenly play the wrong sound – i.e. that this is not a case of the stray sounds coming from clips which were never there in the first place?
For instance, the clip is playing some dialog and suddenly – while the same clip is playing – you hear sound which should not be there replacing it, or that the clip suddenly goes insanely out of sync?
If this is so, it is caused by the Brass Band Effect in FCP.
This was named by me when I was editing a sensitive drama some years ago and people would be talking on the track in hushed tones. All of a sudden a large brass band would blare forth for a while and then disappear.
The Brass Band effect is caused by an ancient bug which occasionally reappears if you import audio either recorded at the wrong rate, or in some format which FCP does not particularly like that day. The only cure is to export the offending sound clips as QT audio files and then replace the bad clips with the QT files.
If this is indeed a case of Brass Band syndrome, this corruption will continue randomly until you do something about it.
Best wishes,
Harry.