Anyhow try setting the app audio to stereo, PCM as many models can't take 5.1 from say HBO and convert it to analog audio out. You could debate it (I won't) but so far I find myself looking around the HBO app for audio options.
I have external speakers set to an old Bose Lifestyle III series 2 music system which works fine except when I want to watch an app like HBO or Netflix or etc. The sound will continue of what's on the "regular" station through the Smart TV speakers but nothing from the selected app through the Bose speakers. Note: I can watch the apps with the TV sound but not the Bose speakers.
Here's the set up. Samsung Smart TV un48h6350afxza, Bose Lifestyle III series 2 music system ,ATT Uverse ISB7500 BOX.
CONNECTIONS: Optical from TV to Box, HDMI TV to box, RCA audio out from box to Bose speaker in, Bose speaker out to speakers.