Hijack. It's so popular, it's linked at the top of the BBS

Mark Lord has done an excelent job with it. He has pretty much taken any hacks people wanted and threw them into one complete, and easy to use package. Then he has also added his own features as well. Definitly try it out, even if just for the easy to use Voladj interface (the hack that would "dynamicially compress" the music volume).