I don't see why. Yes, likenesses are (sometimes) protected, but voices ain't.

I've heard too many cartoon characters that sound (purposefully) like famous actors to make me think otherwise.

here's 10 examples

http://geektyrant.com/news/10-cartoon-voices-that-are-actually-impressions-of-other-actors-ergt

(not the greatest article, there's many imprecisions and it failed to mention most Hanna-Barbera characters, like Top Cat which was basically Sergeant Bilko)

Even when told not to do an impression, they do it anyway, case in point Mel Blanc after his car accident, his Barney Rubble changed from a generic "dumb guy voice" to a loose Art Carney impression.

And to bring it back to Ghostbusters, Maurice Lamarche was asked not to do an impression of Harold Ramis but he did it anyway.

By the way, apparently the show runners misinterpreted Bill Murray's comment: apparently he was just amused by the fact that Egon was the only one that sounded similar to his real life counterpart. The comment was interpreted as "doesn't sound like me" and Music was fired.