Does your PC not have digital optical audio out? Also any GPU made in the last 10 years should be able to do 7.1 over HDMI. If you have an Nvidia card make sure

Does your PC not have digital optical audio out? Also any GPU made in the last 10 years should be able to do 7.1 over HDMI. If you have an Nvidia card make sure to install the "HD Audio" component when you install the driver.