VirtualDubMod can do that, but keep in mind that in order to input the audio in the program, it has to be multiplexed with a video stream in an avi file. Adobe premiere can change a track's volume as well, and it can handle the audio file individually. There are other tools as well, but most of them are file-type specific. If your source files are mp3s, I can name a few progs that will do the job.