Source: The Adventures of Superman radio show (1940-1951)
Type: Other media
So who was the voice of Jimmy in the halcyon days of radio serials? Two people, actually. Radio actor and stand-up comic Jackie Kelk was first, playing the role from its inception to 1947(!). Jack Grimes took him the rest of the way to 1951, and then reprised the role in the 1966 Filmation cartoon The New Adventures of Superman, adding another 4 years to his stint as Jimmy's voice (he also played Chim-Chim and Sparky on Speed Racer). His was a squeakier tone, as if Jimmy was going through puberty, while the steadier Kelk gave it a more standard heroic and youthful breathiness.
Not sure which I like more, not if I'm obligated to choose.