It usually can decode it. If not, you check your mute or volume settings in the usual places and then maybe it didn't record audio.
If it didn't add audio, it is not recoverable.
Bob
I have just interviewed a client using the webcam recording function of movie maker. I'm now back to review and edit the video and I don't hear any sound..!
I tested it with a short 5 second video of myself before I left and everything was fine.
If anyone can help me with recovering the audio for this interview I'd be forever grateful!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic