USB ports do everything except export music

First thing is to open 'System Preferences' from under the Apple menu at the top left of the screen and click on 'Sound' in the row of Hardware icons. Click on the 'Output' button, the middle of three buttons at the top of the window. If you have a USB output device plugged into a USB port it should show up there and you'd click on it to select it for output.

Similarly, if you have a device with optical input plugged into the headphone jack it should also be in the list.

I suppose it's possible that the USB audio output of the iMac could be not working while the rest of the USB functionality is. It's also possible, of course, that the USB input of the Pop Pulse isn't working while the optical input is. You'll need a different computer or a different DAC to sort that out, though.

I wasn't saying that I think the USB audio output of the iMac isn't working, in fact I think that's unlikely, but you won't be able to figure that out without testing with another computer or another DAC.