[Scott Spicoli]

Compressor needs six discrete tracks (SMPTE order) as stated by warren to create Dolby 5.1 .ac3 files.

for future projects there ae different mics you can use that will help to create dolby pro logic II and discrete 5.1 stems for dvd broadcast etc.
check out http://www.holophone.com i use the H4 SuperMini it encodes dolby surround to stereo onto my video camera then the dvd plays pro logic II surround sound its awesome

hope this helps in some way