Make sure your (Samsung) TV is NOT outputting "DTS Neo 2:5" sound (and or check other modes on your TV sound output) if you want the subwoofer on your Samsung HW-Q910B soundbar to work. At least that was the case for me.
More Details:
I got the subwoofer and the back channel speakers to connect / sync with the soundbar; I had solid blue light for both. Great, yeah? Nope; after listening, no sound out of the sub. Using YouTube 5.1 tests, the subwoofer (LFE channel) wouldn't put out any sound at all. For the rear speakers, either no sound, or only one of them working (more on that later).
I tried 'everything' on the soundbar components side. Loading the Smarthings app and updating firmware. Following the directions to ensure the firmware update was pushed to the subwoofer and rear speakers. Resetting the soundbar. Unplugging everything and starting over from scratch. Setting the sound modes to Standard, Surround, etc. Ad naseum. Nothing worked for the subwoofer.
Then, finally, I went into the TV sound settings, looked at the format being output, and it said "DTS Neo 2:5" (DTS Neo 2.5 ?) After a quick search of that, seeing it was Samsung's special proprietary format, I figured maybe that format is something that my 'old' TV didn't really properly support, or maybe one that the HW-Q910B's sub-woofer doesn't support. Either way, I decided to try w/something 'simpler' like Dolby (and ensure PCM format; seems like I read something about that in the manual). Anyway, after switching away from DTS Neo (and maybe some other mode?) voila! The subwoofer works!
Now, for the rear channel speakers, I had only one (the right channel) working. They have a proprietary connector into the receiver box, and speaker wires that are far too thin IMNSHO, so I couldn't easily try other wires to see if that was the problem. They color-code the left and right speakers. Why? I figured maybe I'd hooked them up wrong, and there was something 'special' about the connector or speaker that wasn't obvious (I don't think there is). Long story short, after trying permutations of wires in the 'correct' and 'wrong' color plugs and speakers, nothing worked. However *ensure you have 'surround' mode set* on the soundbar / remote, or the rear speakers may not work. Also, what ultimately solved the non-working speaker? Jiggling the wire again, after I'd connected/re-connected the speakers multiple times. Gaah! So definitely try testing various positions for the speaker wires in their clip connectors at the back of the speaker.
