The orange could very well indeed be a digital audio connection, but would most likely be coaxial digital and not optical. It depends on the sound board built in to the media center computer. It may support optical too believe it or not, but you'd need a 1/8" optical to regular toslink adapter to use it if it supports it.

Provided it at least supports coaxial digital, you'll need to pick up a 1/8" mono to RCA type cable, and you'll connect it to the orange coaxial digital input on the receiver. Be sure to check your receiver for digital input assignment, and you may need to activate that output using software in the computer.  

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