Stevie Wonder playing the talk box: YouTube
The talk box works on the principle of reproducing sound from an amplifier and directing it into the mouth of the performer. The performer's lips and vocal cavities (mouth, throat, and larynx) further modulate and shape the sound. The resulting "talking guitar" output is then fed through a microphone and from there is amplified through the PA system or sent to the recording console of the studio.
Instead of a speaker, the talk box uses a compression driver. These drivers are actually designed to be used with PA systems to reproduce high frequency sound. A length of vinyl tubing is inserted into the open end of the driver. The other end of the tubing is placed in the performer's mouth to modulate the sound.
The talk box is connected to the speaker output of the guitar amp. If a single amp is used, the output from the talk box is plugged into the speaker cabinet. A switch on the talk box sends the guitar amp output to either the internal horn driver of the talk box, or direct to the speaker cabinet.
Note that a microphone is an integral part of the talk box effect, since without it, the effect cannot be heard over the din of the average band.
+Talk box: Blamepro l National Talk Box Association
+Stevie Wonder: History of Rock l IMDB l Official Site l Motown
+YouTube: Video find goes to my man 2Liter
+Toothpaste for Dinner