Audacity is one tool for windows computers that lets you split and combine tracks, for example, and it's free. I don't know what all is available to professionals, but I wouldn't doubt that there is stuff that will let you do the audio equivalent of photoshopping my head on Mel's body.
Audacity, although a nice enough free cross-platform audio editor, is child’s play compared to what is out there for the pros. I am a musician with a just enough experience to be dangerous, and it is almost scary how powerful some of the professional software is. Think of what you have seen photoshopped and apply it to audio only.
I have gone into a studio, sung and/or played a few takes I wouldnt have wasted a cassette tape on in the past, and seen the studio gurus turn it into something fairly listenable.