I faced with problem that was discussed a lot here - when you add/import audio to FCP for 5d2 footage you get audio drift (about 0.1%). To be precise - first I noticed, that when you import back mixdown from SoundtrackPro of original audio + music + folio, there appears drift. I thought that it is nonsense - the very same original audio (of 5d2 files) sent to Soundtrack and then imported back is shorter!
After a while I noticed, that any audio (doesn't matter source nor sample rate) that is imported into FCP appears in DF (drop frame) meanwhile project/sequence settings are NDF 30. And that is the problem!
I found this link http://forums.creativecow.net/readpost/8/1006953 . The thing is - when you import any audio the frame drop settings are affected by Easy Setup! I think this is weird but this is how it is.
So - you create new Easy setting that has 30FPS and set to NDF (non drop frame), then restart FCP and thats it! Next time you import new audio files FCP will set settings from Easy Setup (no more drop frames, no more drift)!