I'm not too certain where this functionality is in Acid, but in Audacity, you can take a look at the manual here http://manual.audacityteam.org/man/bass_and_treble.html. In order to get the audio file into and out of an mp4 container file, you will need a method to demux it. A simple google search has found a program called YAMP (http://yamb.unite-video.com/download.html) which should be capable of accomplishing this task. I have never used this program, however, and cannot vouch for nor instruct you as to how to use it. The documentation in it's website may be of use to you.