Most HDMI outputs on computers are video only. Only a few recent video cards include audio. You didn't say what model the computer is, but I would be surprised if it has audio on the HDMI output.
But almost every receiver made in the past ten years includes optical (TOSlink) and/or coax digital input. That's what you should be using, it will provide much better sound than an analog connection, and discrete 5.1 channels.
But almost every receiver made in the past ten years includes optical (TOSlink) and/or coax digital input. That's what you should be using, it will provide much better sound than an analog connection, and discrete 5.1 channels.