Laptop microphone disabled when headphones plugged in

[Negated Void]

Hey,
I have a Lenovo Thinkpad X1 Carbon (5th gen). It uses a HDA Intel card w/ the "Conexant CX8200" chip, and ALSA.

The problem I'm seeing is that the microphone mutes whenever I plug headphones in. I assume to switch to a microphone included in the headphones? They're just standard headphones with no microphone.

Alsamixer has a "Auto-Mute mode" setting, which doesn't seem to affect this.

Everything else seems to work - the built-in speakers, headphone usage, microphone usage without headphones, etc.

Not sure where to even start debugging that =) Any ideas?

lspci:
00:1f.3 Audio device: Intel Corporation Sunrise Point-LP HD Audio (rev 21)

uname -a
Linux lil-murph 4.13.5-gentoo #12 SMP PREEMPT Wed Oct 11 13:44:18 PDT 2017 x86_64 Intel(R) Core(TM) i7-7600U CPU @ 2.80GHz GenuineIntel GNU/Linux

[bunder]

I believe "auto mute" is meant to mute speakers when you plug headphones in... what about "loopback mixing"?

[Ant P.]

PulseAudio on my laptop has a setting to switch between this auto detection and force use of the internal mic, alsamixer doesn't but there's probably some other way to get at it.