You may have engaged the external input in which case no sound would be output unless you have the external inputs connected. I own a Denon, not a Yamaha, so I'm not familiar with how to change the setting on the Yamaha. On the Denon, you can toggle between auto, HDMI, digital, analog, and ext. input. I have a feeling you might have engaged something similar. All you will need to do is change this setting you will have your sound back.