I use AAC 5.1 all the time on my shield that is connected to my receiver. I would make sure your Android audio settings on shield itself are correct, and enable passthrough in the app if you have not. Also if not, connect shield to receiver so Shield>Receiver>TV, not Shield>TV>Receiver
In screenshot is it direct playing AAC 5.1 and the top right is the signal info from my reciever. 3/2/0.1 which is the 3 “front” channels ( Left Right Center), the 2 side surround channels and the 0.1 is the subwoofer
and a AAC 7.1 file with 3/4/0.1
