select track(s) with clips you want to change, mark in to out covering all the clips you want to change, park on a clip , change audio level as needed go to the fast menu in audio mixer and set level on track – in -out.
without an in to out the option it’s the same but global, or mark just in or out and it will change from or to that mark
http://community.avid.com/forums/p/90355/513393.aspx
http://www.steves-digicams.com/knowledge-center/how-tos/video-software/about-avid-media-composer-keyframe-audio.html