You have 2 controls in this card for loopback mixing the Line input
If you use ALSA loopback controls audio from Line In can be mixed direct to Headphone or Speaker output without passing through PulseAudio.
This does not delay audio like using the PulseAudio loopback, and PulseAudio cannot mistake Line in for for the microphone
Use this command to turn on the Line In loopback and set level to 90%
(change it to 100% once you are sure it won’t be too loud)
amixer --card=1 sset 'Line' 90% unmute
Then use this command to turn the loopback on
amixer --card=1 sset 'Loopback Mixing' Enabled
and this command to turn it off
amixer --card=1 sset 'Loopback Mixing' Disabled
You can also go to alsamixer --card=1 to adjust ALSA settings
Or install a GUI mixer like gnome-alsamixer or qasmixer