However, do I really want to have to buy an adapter, just to get these speakers to work!? I plugged in my old, damned near dead JBL Speakers, and they connected on the “Headphone port”, as expected. It’s not quite the same issue that I’m having, but it’s in the ballpark. If your external speakers still aren’t working, check the instructions that came with your speakers. When in sound preferences, under Output, there is only 'Digital Out', where there should be Built-In Speakers. When I try pressing the volume buttons on the keyboard, a small 'blocked' sign comes up (like on no smoking signs, the circle with a cross through it). Click Output on the right, then make sure your external speakers are selected in the output device list. I currently have no sound from my macbook pros built in speakers. This is just one of those things, thankfully the Apple genius guy was actually pretty darn sharp and figured it out pretty quick. On your Mac, choose Apple menu > System Settings, then click Sound in the sidebar (you may need to scroll down). Plug went fine in the adapter, and then into my USB-C port.

So, the solution, a $9 3.5 Jack to USB-C adaptor. To regain digital audio output, you have to purchase a USB-based device. As it turns out, the design on the 3.5 jack on the ProMedia is such that it doesn’t plug in properly on the curved back of the iMac, but it plugged in fine on the flat side of a MacBook Pro. The 20 MacBook Pros and the 2017 iMacs omit S/PDIF from their headphone jacks, as Jeff discovered. An iMac does not have a digital audio output if what you are exactly referring to is an optical or Toslink audio output. They moved it to a MacBook Pro, worked fine. They moved it to another iMac, it still buzzed. Then they put the ProMedia on my iMac, and the buzzing was there. First they tried a different speaker, and it worked fine. For best results, match the sample rate for the headphone jack with the sample rate of your source material.

In the sidebar of Audio MIDI Setup, select External Headphones, then choose a sample rate from the Format pop-up menu.

Got it figured out, thanks to the Genius bar at the Apple store. Make sure to connect your device to the headphone jack. I just read this posting on the Klipsch Forum:

Macbook pro digital audio out no sound