Hoping I can get some help here. Googling didn't really give me any results.
I have an Nvidia shield connected to an onkyo tx-nr656 in a 2.2 port with an HDMI 2.0 cable. I have audio passthrough enabled on the shield; however, when I play an mkv file with a 5.1 audio stream on either Plex or spmc, I don't hear any voices. I only hear music and what I assume is the rear channel ambience.
Any assistance will be greatly appreciated.
I have a few shows that do this. They have AAC audio tracks, and on my soundbar it sounds exactly how you describe, but on my amp. I would look for an option to force stereo mode, either on the Shield or the amp, which should fix this. Alternatively, you can run the HDMI connection from the Shield to the TV and then pass audio to the amp through ARC, and the TV will force stereo for those specific kinds of videos automatically.
I haven't had the issue on my Mi Box and I don't own a Shield, but I did have the same issue when casting Plex via Chromecast. What I did to solve the issue was reboot it using the Chromecast app and everything worked after that. Unfortunately, it would happen every couple days when I started watching something, but a simple reboot did the trick. I have no idea what caused the issue, but at least it's some kind of workaround.
Is audio passthrough also enabled in Plex and SPMC? What is your AVR reporting it's receiving? I have a Shield TV connected to an Onkyo AVR as well and 5.1 passthrough works fine for me. My Onkyo AVR has these listening mode options I can cycle through. They do things like make everything mono, or force everything to stereo. I would double check to make sure that also isn't affecting things.