FC has a very well known bug that makes drift audio when the sequence when you edit the audio has a different time-base than the default sequence (Easy Set-up).
So if you have your default sequence for 29.97fps, you makes a 23,98 fps sequence and you import audio and edit on the 23,97 fps sequence, the audio will drift because FC considers that that audio has a 29,97fps time base.
The easier fix is to set a default sequence with the time-base you intend to edit the audio, before importing the audio.
For more complicated scenarios:
http://library.creativecow.net/articles/lyon_matt/
rafael
http://www.nagavideo.com