I plugged in a USB headset (SoundBlaster EVO Wireless) into my computer. I started playing a music file using a player application. I opened the PulseAudio Volume Control (pavucontrol) application to choose my headphones as the output. Though the headset was listed in the Configuration tab, it did not appear in the Output Devices tab. So, I could not choose it as the audio sink.
This irritating problem can be solved by just killing pulseaudio and opening pavucontrol again:
$ sudo killall pulseaudio
Tried with: Ubuntu 14.04