Comment # 6 on bug 1085710 from Takashi Iwai

(In reply to Daniele Tombolini from comment #5)
> (In reply to Takashi Iwai from comment #4)
> > (In reply to Daniele Tombolini from comment #3)
> > > (In reply to Takashi Iwai from comment #2)
> > > > Try to disable "Loopback Mixing" mixer enum.
> > > >   % amixer -c0 set "Loopback Mixing" Disabled
> > > > 
> > > > This often leads to such a glitch.
> > > 
> > > Yess but without loopback, rear Linein does not work and I need it !
> > 
> > Do you really need it?  It's a direct analog-loopback route from the input
> > (e.g. line-in) to the headphone or such, and it has nothing to do with
> > recording.  (So it's just a sort of "monitor" route.)
> > 
> > The recording works fine without this control.  Its volume is controlled
> > solely via "Capture Volume" in addition to "Mic Boost", etc.
> >  
> > If you really need this monitor route, then such a glitch is unavoidable. 
> > This is the hardware feature of the codec chip.
> I need the linein because I share the speakers with another PC. I just need
> to hear incoming sound. Is this doable or not ?

You can do it via PulseAudio by routing the signal in software, not in
hardware.
But of course it'll cost more CPU power.  Essentially it works as playing the
recorded sound.

> Is doable without jack sensing ? I tried but when I select headphone as
> output I still hear sound from speakers too.

Sure, the jack detection and the auto-mute can be turned off as long as you do
mute/adjust volume manually.  Simply disable via "Auto-Mute Mode" mixer.

  % amixer -c0 set "Auto-Mute Mode" Disabled