A common problem on the Apple forums from Final Cut Pro X users has been the inability to hear audio from DV files imported from iMovie.
iMovie creates raw DV files (.dv) which contain video and audio data but no metadata or other information. It's essentially a DV QuickTime without the surrounding QuickTime structure.
This means that Final Cut Pro X can't find the audio because it's mixed in with the video data and there's no index or track structure to point it in the right direction.
To get around this, you must convert .dv files to QuickTime movies. Here's how to do it:
1. Open up the .dv file in QuickTime Player 7, which should be in your Applications/Utilities folder. If you don't have it on your system, you will need to install it from the Snow Leopard DVD.
2. Go to File > Save As and make sure the option to make a self-contained movie is checked.
3. Click Save. It shouldn't take very long because it is restructuring the data rather than recompressing it.