If you're in Premiere, you can expand the aduio track then adjust the volume rubberbands. You can also adjust balance as well.
If you're using something other then Premiere, you can download <a href="http://audacity.sourceforge.net/windows.php?lang=en">Audacity</a>. You can the click on the button with the arrow with a dot next to it. Then adjust the points on the timeline to make a fade in or fade out.
