Yeah, I noticed that as well. The NT drivers work, although they are extremely fussy about IRQ's, and such unlike the WDM drivers will totally lock your system in win2k unless they are set just right.
I believe I have found the reason for all this on M-Audio's new drivers
page:
http://www.midiman.com/m-audio/drivers2.htm
Listed under Windows 2000 Notes is: "Limited to 16 bit stereo input and
output on
non-ASIO based apps". So it appears Winamp is a non-ASIO based app,
hopefully
M-Audio will get this input problem fixed in one of the future builds.