After desperately searching for a solution to the Debian install on my old 2012 iMac not producing sounds through the headphone jack connected speakers, this provided the solution:
To fix this problem, you have to load snd-hda-intel module with model=imac27_122 option.
To do this, create a new file (if similar conf file for snd-hda-intel module exists, use that one) for example /etc/modprobe.d/sound.conf with following content:
options snd-hda-intel model=imac27_122
and reboot, that’s all.