I've generally noticed that the 5.1 mix is the default (but if I'm watching a TV series, e.g. MAD MEN, when I select 2.0, it "sticks" to that choice). But you should ensure you're selecting content that HAS a 5.1 audio stream.
If you have a BD player or a device that connects to your Hi-Fi via HDMI (e.g. iPhone, iPad, iPod Touch) that can be a comparison. E.g. Netflix or HBO Go on Chromecast or an iPod. I.e. I've watched the same movie on my TV's Netflix app, then watched it on my Chromecast. DD 5.1 from the TV's app, DD+ 5.1 from the Chromecast.
ADDENDUM: I can't rule out that the TV has some issues based on some testing I just did. The S/PDIF configuration output options are PCM, DOLBY DIGITAL and DTS. From digital broadcast TV, DD 5.1 works fine. With HDMI inputs selected, only PCM is available; same with Smart Hub, PCM is the only option available in the settings.
Using WAR OF THE WORLDS GOLIATH as a test stream, with my Chromecast plugged into HDMI IN 2 of my Yamaha receiver, Netflix offered English DD+ 5.1 as well as English Stereo when "casting" to the Chromecast. I then plugged the Chromecast into the TV's HDMI3 input, and the Netflix app only offered the English Stereo audio stream on the same movie. Seems the Chromecast determines supported audio formats, then informs the app on the device. The TV's HDMI ARC return audio was identified as PCM 2.0 by the AVR. I then plugged the same iPod Touch directly into HDMI3 of the TV (same port the Chromecast was in), using an Apple Lightning Digital AV adapter (MD826ZM/A). Again, the TV only passed PCM 2.0 audio to the AVR, and the Netflix app on the iPod Touch only offered a 2.0 soundtrack. I plugged the Chromecast back into my Yamaha AVR, and the app now offered DD+ 5.1 for casting.
I've my got my arm in a sling post-op for rotator cuff repair, so I can't switch my BD player's HDMI connection. That would be my next test.
This suggests to me that the Samsung TV has a problem with audio streams that aren't simple PCM. I've seen many threads with users complaining of Samsung TVs' digital audio output to soundbars being faulty, so it may be the Netflix app has to output DD 5.1 via PCM, and not pre-decoded. (Someone with more technical expertise in these audio stream formats, please jump in and correct me as needed.)
Last edited by ChromeJob; 10-06-2014 at 12:24 PM.
Reason: Further troubleshooting with Samsung ES6150 TV