<HTML>From <a href="http://www.winamp.com/nsdn/winamp2x/dev/sdk/FRONTEND.H">www.winamp.com/nsdn/winamp2x/dev/sdk/FRONTEND.H</a>:
#define WINAMP_VOLUMEUP 40058 // turns the volume up a little
#define WINAMP_VOLUMEDOWN 40059 // turns the volume down a
So, simply create a SendMessage command with MessageId = WM_COMMAND, WParam=40058 or 40059, and LParam can be empty (or 0)
You'll probably want to bind these to WHEEL_UP and WHEEL_DOWN gestures, or something simple to draw since each "command" will incrementally increase or decrease the volume.
-- Jeff</HTML>