I don't know whether or not the voice actor doesn't want credit. The problem with voice actors is it's hard to even get a clue about age or gender. For all I know the voice actor could have died between the time Inside-Out Boy stopped making new stories and imdb.com launched.
Sometimes for old media finding references is difficult. I had the same problem trying to credit an actor when I reviewed the only public domain episode of the 1950s Adventures of Superman TV series.
Yeah. I bet it can be hard. Right now, I'm trying to find a local TV talk show or something that aired an episode in Oregon in January of 1983 featuring my brother, my sister, my dad, my mom. I was born in 1985, so this was before my time. National television programming seems to be easier to find than that of the local shows.
Finding a citation is one thing but an entire episode from an early '80s local broadcast is a whole different task. Yeah.
Maybe you've already tried but if I were you I would make a post about that on the Lost Media Wiki's forum. Maybe someone who specializes in lost media can help you track that down if any copies still exist.
Good idea.