Havnt really played with this yet but pretty sure you can do it via the codec filters ..
If you have cccp http://www.cccp-project.net/ or K-lite full http://www.free-codecs.com/download/...Codec_Pack.htm (only ones Ive used)
You can configure the FFDshow audio filter and how it outputs to your sound card. Pretty sure there is a winamp2 section in the mixer settings as well
but as I said havnt looked into it so not 100% sure it will do what you want...only other way is setup the sound to output to 2.1 an split the left right with a Y cable