tosh9i

AC3, that's good, that's what I use. Open AC3 Filter Config > Gains > from there you should see L R C and so on, those are your speakers. Note: there are Input and Output gains, I don't have a clue as to what the difference between the two are, so I make the same adjustments on both the input and output gains.

Bottom line, play something (mp3) while adjusting those gains (volumes) until it "feels" right.

Also, on the Mixer Tab, you'll see Output Format, make sure 5.1 is selected