Check out Deck by BIAS, same people who make Peak which is a two track mastering program that I and a lot of people use.
Deck is their multitrack editing/arranging program- sort of like their version of ProTools. Not sure about the light edition, but the full edition is 5.1 capable.
www.bias-inc.com
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SECONDLy, is there such a thing as inexpensive 5.1 mixing software? that
would be fun.