Comment # 4 on bug 911615 from Takashi Iwai

(In reply to Jon Nelson from comment #3)
> (In reply to Takashi Iwai from comment #2)
> > Are you using PulseAudio?  If yes, it's PA who changes the volume.  And the
> > culprit is rather the part:
> >   "Also, sometimes it thinks headphones are plugged in and they most
> > assuredly are not."
> > 
> > How did you check this?
> 
> I went to 'pavucontrol' and it showed that the headphones were the output
> port chosen (which is weird, since I never use them).

It's difficult to know who chose it, then...

You can track the even via "alsactl monitor hw:0"
If the driver really issues a bogus jack event, you must see there.