Clipping in GarageBand

I was recently asked to record a cello and piano duet recital for a friend, which I did this past weekend. I had previously used GarageBand on my MacBook Air for voice recording, but never for a musical performance. I ran into a few glitches, but by far the biggest problem is the clipping of some passages (either the piano or cello or both) because the input gain was set incorrectly on the mic (too low as it turned out, as the sound check used to set the input level was not the loudest either instrument would play in the performance). I’d really like to give my friend a decent-quality recording, but I don’t know how to fix this particular problem on a track that is already recorded. Obviously, since it was a public Christmas recital, I can’t go back and record the music correctly.

I recorded using a Samson G-Track Pro mic connected directly to the MacBook Air, and each piece was recorded as a separate track in GarageBand. All recorded tracks were recorded with the mic set to mono, and GarageBand set to record both channels, as I was only able to pick up the left channel about 60 seconds before the performance began, and I didn’t have time to fix it. This means I can’t mix the two instruments, but at least my friend will get her recording (I hope). Thanks in advance for any help or suggestions.