Before Graham Mackenzie joined forces with AgesandAges, he was performing as proGrammar, a rapping, singing, beatboxer who has been making beats and recording since the early 90s. On his site, there are literally hundreds of streamable songs, including some nice covers, and more than a dozen albums to choose from. But did you know proGrammar also produced a couple segments for Sesame Street while in New York? Oh yes he did.
Here's a clip where the letter M is used to make music with your mouth. ("Anyone can do it!")
h/t Theodora Karatzas