I have a ROM (not sure where from) called
Mark's Sound Demo. I asked about it on here some time ago, but nobody seemed to know where it came from, or exactly how it works.
But playing around with it, it produces some really excellent sounds. I think someone speculated it may have been an Imagic dev tool, but I don't recall who said that.
Anyway, you turn on the voice(s) by changing the fifth character over (the small one) to an "S", and holding the fire button and moving the joystick left or right (an "On" will appear on the fifth row down). Select an item by moving the joystick around. Hold the fire button while pressing up or down to change a value.
I'd love to see some documentation on this, and what all of the various values mean (and how they apply to programming the results into the 2600).