USB ports do everything except export music

Am I missing something here? My iMac has several USB ports but none of them will allow me to play iTunes/PM through them. The headset/Toslink outlet is the only way I can run music through to my system. A year or so ago I could come out of the iMac USB port into the Pop Pulse and then on to the system via SPDIF. Now that doesn't work.

Maybe there is a setting that I have forgotten about?
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.
Thank you for the response Sfar! I did as you suggested and yes, USB comes up as an option but no music. The Pop Pulse shows a steady green light when I connect the USB cable from the back of the computer to the Pop Pulse but when music is actually being produced the light should flash. Must be something wrong with my internal USB systems in the iMac since none of the three USB outlets will produce sound. OTOH other USB devices work fine (keyboard for instance).
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 think you are right that the audio output of iMac is not working (USB ports). Thanks Sfar!
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.