I am using firestick in TCL tv the audio input to my onkyo receiver is from the optical SPDIF cable from the tv.
now the tv has built in netflix apps and the audio works fine.
but when i plug the firestick to the hdmi2 port of the tv , there is cranky noise coming from the speakers. then when i switch the input signals , the problem is fine.
From the support forums , i changed the audio settings to dolby digital only. it works fine but i have to keep the volume at 55 max is 62 on the receiver for an optimal audio output. this is not the case with the built in apps on the tv.
Are you giving any simulated 5.1 or a proper 5.1 from the sreaming app. ?