Try Audacity for Mac which is a very simple audio editor. Didn't notice Tomorrow already told you about Audacity. I think we were writing the post at the same time!
Also, You can do it with a bunch of applications which offer you a minimum amount of audio editing such as Logic Express/Pro, Soundtrack Pro, Final Cut Express/Pro (yes, you can do this with FC). I don't know if you could do this with GarageBand.