The Talkbox is a device famously employed by Peter Frampton to create a singing guitar sound. It uses the cavity of the mouth to shape the frequency response of an audio source.
A loudspeaker is set in an airtight box, with a tube coming out of the box and into the mouth of the player. Another tube then connects the mouth to a microphone. This effectively turns the mouth into a filter in the audio path of the system.
As the audio source is played, the mouth can be moved to form vowel sounds in the music. A similar, effect can be created with the much more technically advanced vocoder.