Home Theater Questions? Ask a Technician for Answers ASAP
Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, proposed preliminary tries (if have not been done) is to check/ensure that the AVR-2808CI is configured to produce sound from an HDMI source to the AMP's connected speakers.
Manual Setup / HDMI Setup / HDMI Audio Setup / AMP
Have you considered/tried a RESET of the Denon?
A reset is both a test and a try of a fix. As it implies, a reset would force the unit to revert back to factory defaults removing glitches and config errors but would include memories, presets and user preferences. After a reset, the initial setup would have to be performed.
May have to be done a number of times.
This is very critical - a reset however would not be corrective of an internal hardware concern or a firmware issue; but it would be confirmed. In some instances, it may even expose other problems.
As a process of elimination; since the Denon works "..cable dvr is plugged in to the other hdmi slot and is working well...", then a RESET of the Denon would ensure that its settings are configured to default and expectedly all HDMI sources would behave equally. If there is sound from the cable DVR but still none from the Apple TV, then the Denon is least likely to be the issue which then would point to the AppleTV and/or its related software as the most likely suspect.
Incidentally, have you tried unplugging the Apple TV for an extended period of time? ...the idea is to simulate a soft RESET.
I understand nd no prob; tomorrow or when able.
Additionally, another to try is to temporarily connect the Apple TV to where the cable DVR is plugged in just for test purposes.