5.666 build 3516 (http://forums.winamp.com/showthread.php?t=374929#download) + update patches (http://forums.winamp.com/showthread.php?t=373755#updates) is what should be used (and definitely not the mad suggestion made above to use an 8yr old version).
as there were issues with the 5.66 release (and earlier 5.x releases) which cause issues with recent vgmstream builds which should not be an issue when using the current Winamp release (since there were specific changes made in the final 5.66x builds to ensure vgmstream would work with it).