At any rate, the MAD Plugin purports to offer improved audio decoding over Winamp’s default decoder. It works with Winamp 2.x at the moment, and they add that it doesn’t work with Winamp 3 yet. Their method is described like this:
Internally, MAD represents the decoded audio signal with high precision. Combined with a form of dither, this allows output samples to be calculated with less objectionable quantization noise and more audible dynamic range than most decoders produce today. The result is unusually high-quality output that is a more faithful reconstruction of the signal encoded in MPEG audio than you may have realized possible. […]
Hmm, that sounds a bit like upsampling to me (though I’m not really sure). Still, I don’t use Winamp when I need MP3/Ogg playback — I prefer Quintessential Player instead (its interface doesn’t become super-tiny at 1600x1200).