I just purchased some Klipsch ProMedia 2.1 speakers, and when I plug them into my 2014 27" iMac’s audio output jack, in the Sound System Preferences panel, it shows that they’re connected to “Digital Out”. When I play audio, there’s no output through the speakers at all. The volume is about 3/4 of full on the speakers, and at 100% at Output volume.
If I plugin some headphones to the same audio output port on the iMac, they show up as connected to the “Headphone port”, and I instantly hear audio.
If I plug the speakers into my 2019 MacBook Pro (running Catalina) the speakers show as being connected to the “Headphone port”, and I’m able to hear the audio.
I tried resetting the PRAM, but that hasn’t helped.