The Voice Synthesizer is a small device able to read the movements of the throat and synthesize an artificial voice.
Due to a laryngotomy procedure that Johann Fennhoff had performed on him, Leet Brannis was unable to speak without the use of the Voice Synthesizer. Brannis retrieved the Voice Synthesizer from his suit pocket when he desired to speak to Peggy Carter; she had him cornered at gunpoint beside a milk truck filled with Nitramene bombs.
Using the device, Brannis told her that he was not allied with Green Suit though both of them were missing their respective larynx. He also used the device to tell Carter that Leviathan was coming and that she would not like the future.
Brannis did not expect her to see the future because he threw a bomb at her feet and used the device to explain that she had thirty seconds before its detonation.
Carter found Brannis again at the residence of Sheldon McFee, the Daisy Clover milkman who used the truck. He used the Voice Synthesizer to ask Carter for protection. Edwin Jarvis insisted that Carter use the machine to learn more about Leviathan.