We describe the design and testing of a human machine interface to use surface electromyography (sEMG) collected from a covert location in response audio-visual feedback. Using sEMG collected from the Auricularis Posterior muscle, N=5 healthy participants participated in 6 sessions over multiple days to learn to transition from visual and vowel synthesis feedback to vowel synthesis feedback alone. Results indicate that individuals are able to learn sEMG control of vowel synthesis using auditory feedback alone with an average of 67% accuracy and that this skill can also generalize to new vowel targets. Control of vowel synthesis using covertly-recorded sEMG is a promising step toward more reliable mobile human machine interfaces for communication.