Radio's Superman got his voice from a talented man by the name of Bud Collyer. He almost did not!
Mr. Collyer had already some experience with voicing characters with roles such as Terry and the Pirates (Pat Ryan), Renfrew of the Mounted (a radio series based on an RMCP adventure series written for boys, as the title role), and Abie's Irish Rose (about a jewish man courting an irish catholic girl; he was Abie), on top of plenty of narrator gigs. He did not want to get tapped for another series that would get him typecast as a hero, and may have very well been tricked back into the studio after doing the part of a random, unnamed Kryptonian senator in the first episode of Superman. Eventually, it was settled that he'd do the role in secret, a move that satisfied Collyer and delighted publisher Detective Comics (later DC, of course) who had no problems promoting the idea that it was Superman himself playing himself on the radio.
He was uncredited for his role on the show until 1946, when he finally realized the positive impact Superman was having and gave an interview for Time Magazine for a piece about the show's campaign to promote racial and religious tolerance. Thereafter, he was credited to his role as the Man of Steel.
Collyer not only voiced Superman for 10 years of the 11 year run of the radio show (Michael Fitzmaurice took over for the last year), but would reprise his now famous Superman/Clark Kent voice for all 17 of the Max Fleischer Superman cartoons of 1941-1942, as you can listen to up above. During the show's run, he also hosted game shows, and in 1950 he left the show to do Beat the Clock full time. Later, he'd reprise his voice again in the 1966 animated New Adventures of Superman, although arguably it was a very, very different Superman to portray (Double Trouble here).
Collyer had a extensive background in singing, and his voice had a range he could manipulate on a dime. It was this talent that for a large part made Superman's transformation from Clark Kent easily identifible to the listeners. When he was Clark Kent he spoke in a quiet, unassuming voice; as Superman, his voice deepened an octave and took on a commanding tone of a man who could make you bounce like bullets would bounce off of his chest.
And, yes, he invented Kryptonite, or at least caused it to be created. You'd think it came about to knock Superman down a peg, keep him humbled, present the man of steel with a rusty silver bullet, to completely mangle a simile or two. Actually, the show introduced Kryptonite... so Bud Collyer could take a freaking break once in a while. See, while the shows were pre-recorded, they weren't re-run. In a gleaming example of something that was done absolutely, completely awesome, they wouldn't resort to such shenanigans as today's television industry. Recording was constantly scheduled, and Collyer, as I mentioned, had other jobs going on all the time. Introducing an object that could put the Man of Steel out of action in as plausible as a show abouta a man who could put his fist through a locomotive could muster. Kryptonite could reduce Superman into a groaning mess, a role easily filled by a stand in!
Incidentally, giving Collyer his much deserved vacation time was the reason Batman and Robin were later on introduced to the show. Please re-read that last sentence. If you felt a lump of pure joy rise up from your blackened lump of a soul, then you've decided to follow the right blog and become a fan of the right old time radio show.