This USB headset, new to me:
Bus 001 Device 005: ID 047f:c035 Plantronics, Inc. Plantronics C520
has an issue with seemingly automatic loopback, where - if I do not enable mute on its cord - I can hear the mic in my earphones. How do I disable this? I have checked, and PulseAudio does not have a loopback module enabled so far as I can tell.