What John W. Campbell Sounded Like, circa 1956
This is not directly relevant to Heinlein, but it's one handshake away.
I've written a piece for Tor.com about . It features the voices of John W. Campbell, Isaac Asimov, Ray Bradbury, Willy Ley, and other prominent SF people Heinlein knew well.
Tor has embedded an audio player so you can play it straight from the page. I also included links so you can download an MP3 file to your Ipod, or whatever.
I also discuss an SF radio series, Exploring Tomorrow, for which Campbell served as the host.