Quite a few threads about this in the past.
Winamp + Milkdrop
Milkdrop is an immense visualisation plugin. When i wanted a similar thing for a big house party i got a projector and laptop, set a playlist of random tunes then muted the laptop and hooked it all up. Didnt go with the music being played at all obviously, but it didnt have to, it was nuts =P.
Although you might be able to plug record out on mixer to mic in on computer and have it synced with the music. Havent tried.