15.10 - Center/Rear Chael Sound Stopped Working (HDMI)

Hi all,

I have an Intel NUC coected to my TV via HDMI, with the TV coected to a 5.1 stereo receiver via SPDIF. Until recently, this setup worked well. About a week ago, I stopped getting output to anything other than my left and right speakers from the Ubuntu box. Center chael, rear chaels, and subwoofer all stopped getting their respective tracks from the ubuntu box. Interestingly, there is a bit of sound coming from at least the center chael, but it sounds very faint and almost like some sort of bleedover from another chael.

I have tried a number of things to troubleshoot, but havent had any luck:

--Have tested other inputs to make sure it wasnt a problem with the TV or stereo. Other inputs coected in the same way are working fine.
--Checked the sound settings to make sure nothing was muted. Nothing was.
--In sound settings, if I change the mode to Digital Stereo (HDMI) Output (from Digital Surround 5.1 (HDMI) Output), the audio works, but only in stereo (obviously)
--Checked PulseAudio Volume Control to make sure nothing was muted--nothing was.
--Checked alsameter. This only shows SPDIF, which I understand may be standard for HDMI audio
--Have confirmed that there was no new software installed or updated when this started happening
--If I check the pulse audio volume meter while ruing the "test sound" functionality in Sound Settings, I can see the meter registering the test audio, implying that the system is at least generating the signal, its just not making it back out to the TV/Stereo

Any suggestions on how to fix this, or troubleshoot? Have been googling for several days, with not much I have been able to find.