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Yeah, not Kasem. His cartoon voices always either sounded like Shaggy, or like his radio voice, as obvious in the various bits confirmably him ("Q for Quarter" is Shaggy as professor, Letter B is basically his regular voice).
It's not Arbogast's standard delivery either. It's closer to whoever was the hero, and we don't know, in the Letter V segment, and they're all doing a nasal "hero" voice in vogue in cartoons and comedy for some time (Stan Freberg did it, Dudley Do-Right sounds like it, etc.)