Hober Mallow wrote:
Yep, usually by manipulating his voice to make it really high or really low.
Sometimes his voice was processed, yes; for instance, his Batcomputer voice on the '77 New Adventures of Batman
was slowed down to sound deeper. But a lot of the time he was just varying his own vocal pitch within its natural range, e.g. as Dumb Donald on Fat Albert
or the narrator on countless Filmation shows.
He played Orco and many others on He-Man (a show which, incidentally, used a ton of TOS sound FX thanks probably to TAS).
At the time, there were dedicated companies that provided sound effects for many different shows, using their own standard libraries of effects. Filmation's sound effects and music editing were usually provided by the Horta-Mahana Corporation, although they weren't credited on TAS. TOS's sound effects were provided by Glen Glenn Sound. But I think there was some overlap between the sound effects libraries.