System sounds coming through internal speakers ... and I don't want that

Hi, 2016 MacBook Pro running Mojave 10.14.6. I have an output device (headphones, wired) configured through a USB Audio CODEC (so I can input musical instruments, microphones, etc, and monitor via heaphones). It works great. I can listen to music or videoconference streams or my instruments, no problem. However, the system sounds (like the sent mail or received mail sounds or application error sounds) are still coming out of the internal speakers and only the internal speakers. I have my preferred output device selected via the volume control pulldown in the menu bar and if I check it through Control Panel. This is new: I've had this device for a year or so and this problem just cropped up. I'm not sure if I flipped a bad switch somewhere or this came with a system patch. But I can't figure it out. Is there a CLI config that I can do to force *all* the sounds through my chosen output device?