Volume cutting in and out on recorded tracks.

I was listening to an old song I recorded in GB on a new 17" iMac and the tracks that I recorded live (ie, through the internal mic on an ibook) and the volume shifts at certain points. Almost as if the volume is hitting a peak. Oddly enough, the same thing happens in iTunes ONLY to the songs I've recorded in GarageBand. This has never occurred on my old iBook...could it be due to the fact that I transfered the old GB songs from the iBook using target disk mode? I can't imagine, why...any ideas?