not sure if you got Soundtrack with FCP, but it has a pitch filter (among others) you can mess around with. Since I'm not a music guy, or sound engineer type I was hesitant to use Soundtrack at first, but I found it to be intuitive enough to learn quickly and it now bends to my will nicely. =)
you can export the clip to ST, edit just the audio, and pop it back into your sequence.
edit: just checked in FCP. You might be able to do it right from FCP without jumping between programs.
Check to see if you have this filter in your effects bin:
Effects (tab) >Audio Filters > Apple > AUPitch.
It is the same filter I use in ST. Im not sure if it comes with FCP by default, or if it was part of the ST package (in which case, you might not have access to it from FCP).