like the other two posters, i won't be able to provide you with much help, except letting you know that what you want to do is very possible. there are a number of methods:
1. get a winamp control dll, i think dragonzap made one, and see if it has a function that will call winamp's fade out function.
if that doesn't work, or the dll doesn't support it, you could:
2. set up a timer to incrementally decrease the volume from where it was (see $vol()) down to 0, then use the winamp control dll to stop the song from playing. then, reset the volume to where it was originally, and there you go.
sorry for the not so specific answer/code, but you can take it from here.