I spent way too much time trying to figure this out, so I'm sharing for everyone else's benefit. I have Mint 17.2 Cinnamon installed on a Dell Precision M4600, and at home I use Bose Companion 5 USB speakers connected through a Dell eport plus docking station. The speakers worked fine until yesterday when I disconnected my laptop to go on the road. When I reconnected at home, the Sound Manager could see the Bose Speakers as 'Analog Output Bose USB Audio' but I could not select it as an audio source. Rather than logging out, restarting or cycling the power on my system (which I refuse to do because it's not the way it's supposed to be), I eventually stumbled into the following fix:
$ pulseaudio --kill
$ sudo alsa force-reload
The system will unload the ALSA sound driver modules and then reload them. Sound worked immediately. Hope it helps.