AVR is a Denon 4306. It supports Dolby Digital and DTS. The *only* way I can get any audio out of the stick is to force it to "Stereo". Best Available and Dolby Digital output nothing. No, I am not trying Dolby Digital Plus as I know my AVR does not support it. The rest of my devices work fine - HD DVD, Chromecast, Blu-Ray, Switch, Xbox One, Cable TV tuner - only the FireStick has said issue. Firmware is up to date. Factory reset does not remedy.
NOTE: My Gen 1 Fire TV, the slim rectangular box version - had no issues.