Isn't Hal Smith also a co-starring voice?
He played one of the humans in this movie as he's a well known voice actor who is known as Gyro and Glumgold on Ducktales
It seems that Hal voiced Winnie the Pooh for most of the 1980s, too. I guess that any voices he did for The Hobbit were considered incidental roles--to small to land him a credit, much less co-star status. Sorry.
If you can verify his involvement, maybe you can add an entry under "trivia."
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