In_mad is an alternative mp3 input plugin (which isn't recommended).
Did you only change DirectSound settings (as described in the threads
JonnyMac linked to), those changes shouldn't affect the volume, in fact Winamp doesn't have
any normalizing built in.
Does this only happen in Winamp or also when you play the same file in Windows Media Player? Maybe your sound driver has a built-in normalizer/compressor.