I just stumbled across this 1937 MGM short:
Bars and Stripes featuring Maynard Holmes as "Johnny the Tuba Player" but I don't think he was actually playing his Sousaphone (could have been Country Washburn, by the style - but that is only a guess - IMdb is no help.)
Spoiler - he has a girlfriend!
I did a back search on this -
Chuck Jackson spotted this in 2004, but he got some details wrong and he had no link to the film
