Wow, it would never have occurred to me that signing up through the CBS app on Firestick resulted in different audio capabilities than signing up through Prime Channels on Firestick. You are absolutely correct. I cancelled my current subscription and resubscribed through Prime Channels and...
Mind if I ask what generation Firestick are you using? I'm using the latest generation 4K and definitely not getting 5.1 pass through on any CBS All Access content. I get DD 5.1 on Netflix, Prime and other Prime Channels so its not an equipment problem. Are you sure you're getting true 5.1...