Do you know about the noise reduction capability in Soundtrack Pro? If you have a video with a hiss or strong wind overpowering the voices, bring the clip into a FCP timeline, right click on it in the timeline and choose 'Send to' then Soundtrack Pro Audio File project (not multitrack) and tell it to launch Soundtrack Pro automatically.

Once in soundtrack pro, use the waveform and listen to the clip to find a place where there is only background noise and no vocals. You may have to zoom into the timeline to see the dip in the waveform. Select the area that is only background, then go to 'Process' then 'Noise Reduction' then 'set noise print'. Back in your timeline, select all and go back to 'Process' then 'Noise Reduction' then 'reduce noise'. You will be given a control dialog box.

The noise threshold tells the computer what sound is soft enough to not be affected. So a higher level will affect more of a range than a lower level. (Note that if the background noise exists at the same frequency as the vocals this may not work very well.) The reduction amount tells the computer how much to lower the db level for those frequencies. (If you go too far with this you may wind up with some vocal modulation, so listen carefully to how much you reduce the background noise.) And the tonal slider will let you maintain the bass or treble, depending on how much you lose in the threshold.