[gentoo-user] Sound jack isolations

I tried googling around but no helpful content so far. I have a laptop
with built-in sound. It uses the HDA_intel kernel module. My problem is
that I can't silence the onboard/built-in speakers when I plug in the
headphones to the machine. Muting sound would lead to all jacks and
audio ports to be silenced -- not just the built-in speakers, which I
intend to mute solely leaving sound alive on the headphone jacks. There
are no channels readily observable to differentiate where sound goes and
to which port with alsa-mixer. The behavior I would like to achieve is
one that will allow me to mute onboard speakers while continuing to have
the headphones receive audio signals. Has anyone ever done this? Any
pointers to documentation will be appreciated. 
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