[Scott Newell]

To boost super low audio, click the low audio track to highlight it, then select Effects>Audio Filters>Apple>AUDynamicsProcessor to apply it to the track. Then go the Filters tab in the audio clip and you’ll see a master gain in there. It should provide any boost you need. This is the best solution I’ve found besides the new gain filter in FCP 6.

I believe Apple includes the AU audio filters in versions of FCP earlier than 6. I used to stack tracks, but there are lots of problems you can get into with that method. Plus, if you have to do anything else with the audio track, you only have one to deal with rather than several.

Hope this helps.