It's not possible within Cakewalk. That is, you can mix down both by sending the audio and midi tracks out of the output of your soundcard and onto tape or whatever (by just pressing 'play' in Cakewalk). Unfortunately, you can't mix down audio and midi to another set of tracks with the 'mixdown' function internally in Cakewalk. Basically, what you have to do is to record your midi tracks to actual audio tracks in Cakewalk, and then mix down that way.
It's kind of a pain in the ass, and I wish Cakewalk would figure out some way to get around it, but that's what you gotta do!
Ryan