there's never a guarantee anyone will reply to any posts made, especially considering it's the holiday period and a lot of people are not around / posting at the moment.
other than providing
http://forums.winamp.com/showthread....161361#plugins the only thing i can think off is it might be down to the output plug-in being used as nothing in Winamp should be natively dropping the volume to 20% (though as you say you're using out_ds, then that should be even less of an issue).
unless it's related to your soundcard driver but as something i've never personally seen on a large number of Windows 7 machines i've had access to, something specific has to be going on to cause it and i'd have to guess at a 3rd party plug-in being involved.
-daz