With ACID or ProTools you can create a 5.1 mix. There are other tools as well but not Vegas. Then Export the mix as 6 seperate wav files. Then in Windows Media Encoder select Theater sound for the audio selection. It will ask for the wave files you exported earlier.
Yes, I've done just that, from 6 mono files rendered from ACID4. You have to pick the "multiple file device" as your source in WME. In addition the the AC3 encoder we're doine, a future WMA encoder from us will be able to do surround directly from the app without using WME.
///d@