Comment # 3 on bug 911615 from Jon Nelson

(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).