Mixing Music

This question is very vague. Can you identify a specific technical requirement? Are you talking about mixing as in balancing and processing many channels of guitar, vox, drums into a final stereo mix or are you talking about crossfading? The first question is again too vague to answer. These are free useful resources that might help - http://www.samecoff.com/works.html#Texts

The second (crossfading) is pretty straight forward. Look at the panning examples.

Alternately use http://www.mixxx.org/

Again I may have misunderstood the question is vague.