The M-Audio interface is only a 2 channel device. It will not work to produce 5.1 sound.
What you want to do take the computer's digital connection, whether that is HDMI, optical, or whatever, and connect that to the receiver. Also connect the M-Audio interface to the computer, but go from the M-audio interface outputs to analog inputs on the receiver.
When you want to use the M-audio interface, select that device as the default audio output in your computer's device manager, but when you want to listen to a 5.1 sound, select the computer's onboard digital output, whatever that is.
Most pro-audio interfaces are 2 channel only, unless you get expensive ones.