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). When time and computer stability allow, I'll go through and compile a confirmable list or table for his page, which will help.
It's not Arbogast's standard delivery either (the muttering little man type). It's closer to (or possibly the same as) 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.)