Let me count the ways (iMovie HD)
1. The Mac’s computer audio is turned down or muted
2. The audio master level in iMovie is muted
3. The audio of the clip is muted
4. The audio button of a specific track is turned off
5. The headphones/speakers are turned off
6. The audio is not importing
Too many ways!
7. The headphones triggered the separate level set for the external source and muted the audio.
8. The audio cable is broken
9. The headphones are broken
10. The monitor you are using has it’s own audio levels that can be adjusted on the monitor.
11. The student muted the audio level in iMovie while importing the footage (kills the audio).