Whenever you mix two colors, you always get a new color, at least that's what our eyes tell us.
However, when you mix two sounds, you can still identify them individually, either as aesthetically pleasing or as noise. The same can be generalized for more than two colors or sound. Mixing multiple colors create a perception of a new color, the same does not hold true for sound.
Laws of physics govern both science of color as well as sound. However, I think evolutionarily human ear is more developed than eye as an organ. That's why we cannot distinguish two colors being mixed, however, we can identify two different sounds. May be ears have evolved over a longer time scale than our eyes have!!
[Note on 01/29/2012: The above conclusion seems to make sense with my newborn baby, who can hear fine but eye-brain co-ordination is still developing]
That brings me to the following questions:
There are colorblind people but is it possible for someone to have defective eye or gene somehow so that he/she sees the world in black and white?
Why there are only 3 primary colors(Red, Blue & Yellow)?
Why do our eyes can detect only 24 frames per second? Anything faster than that will detect in brain as moving object.
