Hello I am running a digital optical audio cable out from my vizio smart tv to a 5.1 onkyo receiver. When the receiver is in stereo sound setting, I have no issue. However, when I switch to 5.1 sound, regardless of if it is dolby digital or not, I get a buzzing sound which sounds similar to white noise. Any ideas on what could be causing this, or solutions for me? Thanks!
Since you are using an optical connection and it only happen in 5.1 I suspect the receiver has a problem.
To be sure disconnect all your sources and try it then.
Onkyo has had some issues with HDMI boards that can cause audio problems. If you google the make, model + recall for your receiver you may be able to get a free repair out of warranty.