From Muppet Wiki

Thad Mumford (b. 1951) was a songwriter for Sesame Street. One of the songs he wrote was "The Ten Commandments of Health," in which he also performed the voice of the lead singer, Dr. Thad. Every Body's Record also credits him as the voice of one of the Monotones in the song "With Every Beat of My Heart." He has also worked as a writer and producer for many television series including The Electric Company, and helped pen the script for the highly-viewed final episode of M*A*S*H.