I had used the HDMI port to play a movie onto the TV and of course I used the HDMI as the sound output. By unplugging the HDMI cable from the TV, Banshee no longer was able to produce any sound through the built-in speakers, yet I had no way to release the output from the HDMI.
The solution was to plug my laptop back into the TV via HDMI then set the sound's output back to the built-in speaker before unplugging the HDMI.
System: Lenovo Ultrabook 15 Flex, Linux Mint 18.1, Banshee 2.6.3
