*ignoring Frink*

here's some HB characters whose voices are basically impressions:

Yogi Bear = Art Carney
Jabberjaw = Curly Howard (and a pinch of Rodney Dangerfield)
Wally Gator = Ed Wynn
Snagglepuss = Bert Lahr

although apparently, as the last case seems to imply, even if voices aren't copyrighted, their unauthorized use might potentially lead to problems:

Wiki sez:

When the character of Snagglepuss was used for a series of Kellogg's cereal television commercials in the 1960s, Lahr filed a lawsuit, claiming that the similarity of the Snagglepuss voice to his own might cause viewers to falsely conclude that Lahr was endorsing the product. As part of the settlement, the disclaimer "Snagglepuss voice by Daws Butler" was required to appear on each commercial, thus making Butler one of the few voice artists to receive a screen credit in a TV commercial.

oh, yeah, I forgot: Warner Bros characters which were "ripoffs" of (well, more inspired by) previous characters

Pete Puma = Frank Fontaine's "Crazy Gugghenheim", which was later re-used by Dan Castellaneta in the Simpsons as the basis for Barney Gumble
Foghorn Leghorn = Kenny Delmar's "Senator Claghorn"