A sound to light visualiser does one of two things:
1. drives one galvo with bass, the other with treble (this is how my first laser show worked, with a 40 milliwatt argon laser!)
2. drives the galvos like an oscilliscope, fed via winamp. A sawtooth is applied to the horizontal axis, and the music signal applied to the y axis. Thus making a (low resolution) oscilliscope. This is how my second one worked. An oscillator generating a sawtooth feeding the X axis, and the music signal (I think it was the right channel I used
) feeding the y.
DMX boards can be used to produce laser shows - these contain from 40-200 patterns on board, and change according to the music. They get boring after a while though. These can be hard to find, unless you get your hands on a defunct projector. Usually the lasers or galvo amps go in the cheap chinese ones, but the dmx boards are fine.
The best way to have fun with a projector is to control it from a laptop PC or desktop PC and syncronise it to music using software like spagetti and a dac, or a hardware software combo like Pangolins Flasback 3 and Quickshow. The hard/soft combos get mighty expensive though! It does give you more options though - such as the ability to activate cues on the fly, and also the ability to make your own shows to your favourite tunes.
Whatever your choice (soundcard dac, spaghetti, Pango Quickshow etc) DO NOT TOUCH ISHOW - many folks will agree it is worse than crap!
I have a frame streamer express 2.0 which is a dac that works with LFI, mamba, he laserscan, spaghetti and a few other programs. I will be upgrading to a pangolin quickshow in the next few weeks, as it opens up a whole library of artwork and laser shows to me. The cost of that system is not just the hardware and software, but also the content that you get access too.
BTW where do you intend to use this projector - 2W seems like an awful lot of power! Hope you have saftey goggles on when working on it with the laser "hot"