I just bought a Denon AVC-S660H and set it up with my speakers and all devices. There was fine sound during all the tests, but after setup was over there was no sound in the speakers from neither of my sources. The troubleshooting guide in the user manual didn’t help.
I had to find the answer in a random YouTube Video: Go to the HDMI Setup in the Video menu and disable HDMI Control. Boom!
Note: This solution may work for other Denon receivers as well, but it may not resolve your issue.