I made a little app to show how you'd do this. Because mp3s are big, I used colors instead: drag a slider to the left and a square gets redder; drag it to the right and the square gets bluer; put the slider in the center and the square is a mix of red and blue.

The trick is that I used two squares, a 100%-red one and a 100%-blue one. The blue one is on-top-of the red one.

The slider adjusts their alpha properties, so if the blue square is partially transparent, you can see the red square through it.

Dragging from extreme left to extreme right changes blue's alpha from zero to one. Meanwhile, it does the opposite to red's alpha, changing it from one to zero. When the slider is at the half-way point, both alphas are .5, making purple.

In AS 3.0 all percents are expressed as numbers between 0 and 1, not between 0 and 100. Volume is also controlled by a percent, so my code could easily control two volumes instead of two alphas.

Here's a link to the file:

http://www.grumblebee.com/stuff/cow/twoWaySlider.fla
Author "After Effects Expressions"

Co-author "After Effects For Flash, Flash For After Effects"

Artistic Director, Folding Chair Classical Theatre, Inc

Senior Flash Developer, Zeitbyte.com