linux – loopback/listen to Line In, ALSA/Pulseaudio

I have 2 computers, and one set of headphones. When I used Windows on my desktop PC I could simply select Listen to and it would pipe the Line In input to the default output device. I don’t see a way to do that on Arch Linux.
I looked at alsamixer, but found it too difficult to use.
I tried using pactl module-loopback, but it looped my microphone, probably as it was set as the default in GNOME audio settings and/or pavucontrol
Pavucontrol allowed me to mess with the source of module-loopback, but it only allowed “Analog Stereo Duplex” or “Ellesmere HDMI Audio” as options.
I want to use my microphone for calls, but also have the line in jack be played back.

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