Don't even waste your time trying to mi/matrix in SF. That's what Vegas is for. Trust me, I've used both for a long time (mostly for video work, but more recently for AUD only work). Edit and apply effects and whatnot in SF, then bring into Vegas on different tracks, get them lined up and in sync, and then play around with the volume of each source. When you're done, render out to one stereo wav file. You can also just bring into vegas first, and launch into SF from vegas at will.