This site is NO LONGER being maintained for the "ALL NEW" site that talks about the new iMovie 11 please visit TheUnofficialiMovieFAQ.com
Are you sure you did not turn it off? Look over to the right, are the check boxes enabled? Each checkbox refers to a audio track. Yes, the video is considered and audio track too. You can enable and disable these at anytime. I usually use them to remove music, temporarily, while I am checking my voice-overs, etc.
If the " Video Play Through Camera " is enabled it sends all audio through the FireWire port and mutes it on the computer. This is a common problem with people thinking they have no audio or an audio issue. Turn this off in iMovie preferences.