I have connected a digital optical cable to my pc and then to my a/v receiver, but I do not have sound. The connection is to my motherboard's sound card. My gpu does not have a optical connection, and I do not have 25ft HDMI cable. I do not know what I have done wrong. Is something wrong with my sound card or a/v receiver? My pc uses realtek audio, and I have made it my default in sound settings so I don't know what is going on. This is a pc I built myself about a year ago, and now I have grown tired of using headphones and want to use my 7.1 home theater system. Any ideas on how to fix this?