Unstable motherboard audio amplifier? Or dying sound card?

Hi,

I'm having a troublesome experience with my computer (see at the bottom of this post for the detailed specs): each time I increase the sound level past 47% using the task bar icon (on Windows 7) - which I rarely do - the background noise is amplified and diverges, as if the integrated amplifier had become unstable. The problem appeared yesterday as far as I know.
I've tested this phenomenon using different headsets of various impedances, and it doesn't seem to depend on the load connected to the system. Neither does it depend on whether the headset is connected to the front or the rear panel (through a simple 3.5 mm jack).I built this computer 5 years ago now, and I'm quite sure I never experienced any problem like this one. However, when the Realtek HD audio manager is enabled, I regularly get pop-ups telling me that a cable was (un)plugged, regardless of the cable/headset/speakers tied to the computer. Also, several of the audio jacks appear as plugged in even if they don't (though I don't know how accurate this software is). Currently the computer isn't grounded (as it has mostly been for the 5 past years), which may account for the background noise, but not for the wrong indications or the instability of the sound level. Of course, there's an easy workaround to this: not pushing the sound level past 47%.
What bothers me is that I get more and more clues that there might be a problem with the motherboard (the USB controller seems to have problems too), and that I might blow up my headphones if I leave the computer while they're plugged in (as I don't know whether the 47% threshold will hold over time or not). Do you have any idea where it could come from? Any solution? Any help would be greatly appreciated!

Please help.

I did not find the right solution from the internet.

References:
https://arstechnica.com/civis/viewtopic.php?f=6&t=1395385
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Thanks!