OK.
Get a good stable clean +5V power supply and three 50K to 100K pots.
Connect terminal 3 of each pot to the +5VDC, terminal 1 to GND and terminal 2 to the analog mod input of each laser. Now you have TRUE analog color control.
You can also add some momentary push buttons for instant on and off of each color.
Get some linear slide pots. Play it by hand.
Forget about the DC modification for now and use the stereo output of your sound card as-is.
Dean's application LWave should be able to make some stuff that will look pretty cool; even if it isn't a perfect DC representation of it.
You will at least have some pretty colored pictures.
James.