I just purchased the retail version of MS3 and in going through the instruction book, it mentions that there are three video tracks, I see those, and three Audio tracks, I only see two (Voice and Music).
The three audio tracks, in the order they appear from top to bottom are:
Voice
Music
Sound Effects.
All three can be used for any audio tasks you have in mind, despite their naming conventions.
You might only see two because your tracks window is scrolled all the way to the top. See if you can scroll down using the scroll bar thumb on the extreme right of your MS window.