My last resort is to try a fresh install of LibreELEC just incase any settings have been changed that have caused this.
That is the last thing I can try.
Yes, that's my final thought, too. During the updates an analogue setting maybe got lost. Analogue output is not a very popular feature.
Here's a USB Audio Adapter that does not hog room from your usb jacks
Yes, that's cheaper, but he was talking about "aesthetics of the case". So let's see if he invests the extra money for that reason. Just an HDMI cable looks better for sure.
Hopefully the case has no additional pass-through resistor at the audio jack.
Tried your guisettings.xml and it's exactly the same, still low audio via analogue unfortunately.
OK. Next theory: The analogue audio hardware has been damaged, and your other OS compensates it by doing extra amplification.
To give you a more practical advice: You could buy an HDMI audio extractor.
Benefits:
...audio keeps getting cut without any CPU spike.
That can indicate a bad connection. HDMI has different strands, maybe just one of them is bad. Try other cables.
You do autostart scripting to make your case work, right? If my guisettings.xml don't work, deactivate the autostart scripting for a test run.
Thanks for the feedback. It looks like we are still dealing with master volume control, not just the analogue part.
I've been uploaded my current RPi guisettings.xml. Give it a shot.
I really don't know whether analogue volume control affects your output, but maybe it's worth to check. Read here.
I found the solution to edit analogue volume. 
Edit the file /STORAGE/.kodi/userdata/guisettings.xml.
Change the XML content of "audio" (bottom of the file) like this to get max. analogue volume:
<audio>
<mute>false</mute>
<fvolumelevel>1.000000</fvolumelevel>
</audio>Thanks for the test.
- HDMI is not even an options on my LE. I have 3 ALSA HDA entrys (Default, Intel PCH ALC283, and Intel PCH MJI) + PULSE Blootooth
Out of those, the only one that produces any sound is the Intel PCH MJI one.
Wow, that's weird! It could be the reason for low volume.
Can an Intel NUC user please verify that HDMI is a default output option at the audio settings? I always have this option on my RPi.
My first guess is that you should check the HDMI cables. Use "HDMI High Speed" cables to be sure.
Sometimes the cable contacts (internal soldering) go bad after some years. I already had this issue, and had to replace one.
PS: Be sure to push the plug to the max into the RPi.
OK, on my headphones LE is loud enough. I have set the audio channels to 2.0. Maybe it affects analogue audio, too.
Some brainstorming:
- change the wiring to NUC -> TV -> AVR
- select "HDMI" as audio output device on LE
- only use 2.0 channels on LE (this LE option is a bit misleading)
Maybe one of those works. Try combinations, too.
Go to "Settings -> System -> Audio -> Audio output device" and select "PI: HDMI and Analogue".
Proved by myself.
Another theory: maybe the "Intel burst technology" for temporary high frequencies on Celeron is not supported.
CPU Speed: 798 MHz (most of the time) - over a 10 minute period I watched it go from 798 MHz up to 2186 MHz for only a few seconds.
It looks like your system runs fine at ~800MHz, so I think that's the preferred state for LE. It supports my other advice to have a look at the UEFI display settings (HDMI / VGA). If LE has to feed two displays, then ~800MHz could be short.
Will do.
But if I remember correctly this issue has been present for years.
Thanks. I have seen a lot of people here in the forum, who run Intel NUC's without issues. That's why I'm curious about your one. The 9.1.x development is mainly targeted on RPi4B, which currently has heat issues. Maybe devs play around with CPU frequencies at the moment, so I suggest 9.0.2.
As I already mentioned: Do the tests with a stable release - 9.0.2 is the latest. Expect driver issues at 9.1.x development builds.
I had a look at the CPU data sheet, max. frequency (burst) is 2.30GHz, so 800MHz is not close to the max, as I stated before.
The data sheet of the NUC says, it supports two displays (HDMI, VGA). Maybe one of them has to be switched off to relieve the CPU.
