its NOT neccesarily DD5.1 Discrete even if you hear it in 6 channels
Most of the time, the game supports ProLogic2, which uses 6 channels, but its not a true 5.1 signal.. as in the game does not have 5 tracks and a bass-line to send to each speaker...instead it gives 2 signals and the reciever decodes the rest.
If it supports DD5.1, check the back of the box and it will say so. You will see a diagram ... a square with dots at each corner and on on the top line.. that means its a real DD5.1 source. Dolby Digital can be prologic, proogic2, etc etc.
ProLogic2 is a popular choice since you only need 2 real channels. Your reciever can mix any 2 tracks and make it seem 5.1. heck, give it a mono (ONE channel) feed, and if you press the right buttons, the sound will come from all of the speakers. its a mono track, but its on 5.1 speakers.... big difference from a discrete 5.1 source.