Along with Keyframing, take a look at the Audio Mixer in Help, as it can do that too, with a slightly different method of Keyframing.
Also, what version of PrPro are you using? Some things changed between CS 5.5 and CS 6, regarding Audio.
For previous versions (going to add some tutorial links to it now, for CS 6), this article might be helpful: http://forums.adobe.com/message/3472343#3472343
Good luck,
Hunt