I plugged in my Plantronics USB headset, which was previously working with Linux on an older laptop, into my laptop, and the light in the headset went on. However, the sound continued to come out of my laptop's speakers, and no sound came out of the headset. Normally, I would use pavuncontrol to redirect the sound from the application producing the sound to the desired audio output, but I was unable to do this because the Plantronics headset does not appear as an output device in pavucontrol. It appears in the "Configuration" tab as "Audio 655 DSP", but it does not appear in the "Output Devices" tab, and it is not selectable as an output device for an application in the "Playback" tab.
However, dmesg, aplay -l and pacmd list all show that both ALSA and Pulseaudio are detecting the headset. sudo fuser -v /dev/snd/* does not show any rogue applications monopolising any ALSA device.