Could be a lot of things. When you drop it on a timeline, is there a red line on the top of the video track? Do you hear beeping when you try to play it? Then FCP needs to render the audio. If this is the case use one of the easy set-ups for FCP next time you digitize, make sure it is for the “non-controlable device”. Is it dead silent? If you can hear a very faint sound it maybe that your audio is 180 degrees out of phase. They end up cancelling each other out. Delete one track of audio and pan the remaining to the center. How about your audio levels? Do they read anything? Will it play in the veiwer and not in the time line? Timeline settings could be off. Get info on the file in Quicktime, what dose it say? DV NTSC 720X480 Video 29.97 fps 16 bit 48kHz audio? What is your audio output set to in FCP? Firewire port? Audio out? These are just some options, but without more info it could be anything.
Hope this helps-
Matthew
Dual 500 G4
OSX 10.3.5
Ram 1.38
FCP 4.5/AE 6.5/DVDSP3