IS the audio track disables anywhere, or muted? Try extracting the audio with Super and add it as a seperate track.

Hopefully you are rendering into a separate HD. I’m no help otherwise.

Make sure you’re selecting “render all” (option R, I think). Also, open the individual clip in the viewer and verify that there is actually audio (click on the audio tab and should see a waveform drawn). Check settings to make sure audio rendering is set up. Lastly, what format is the native video in? If you have mixed formats, v.6 FCP may be having issues (esp. mixing .AAC or .mp3 files into the workflow). I believe multi-format rendering was fixed in v.7.