Add support for more sound channels

Registered by Renegade

On an 5.1 system, YR uses only the front left and the front right channels as sound output, the others are silent.

##### ADDITIONAL INFORMATION #####
This doesn't need Pro Logic or other complicated stuff. IMO a following script is enough (first is left channel, second is right, values are percentages) (a similar script is used by my ffdshow to remix 2.0 input into the 5.1):
Center (0,75; 0,75)
SideL (0,75; 0)
SideR (0; 0,75)
BackL (0,5; 0)
BackR (0; 0,5)
Back Center (used by old Surround) (0,5; 0,5)
Subwoofer (0,75; 0,75)

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