Given that the only voice credit in his appearances with Von Drake is Paul Frees, it can be assumed that Frees voiced Herman as well.
Herman also appears in Donald Duck short "The Bootle Beetle" (1947), and "Morris the Moose".
In The Truth About Mother Goose, it is established that Herman and the professor have been friends since childhood. He is shown to be a firm believer in fairy tales, which annoys Von Drake. He also frequently says things that annoy the professor, but despite it all, Von Drake cares for his well-being.