What an interesting idea.
Keeper... I have to admit that his voice sounds just like it is. Like a whiny little girl.
Just kiddin' big brother. He's actually got the mellow sort of voice (soft, well-spoken, medium-deep) that girls go crazy over. Except for when he hard-core screams, in which case, Nilp's dead on target with the Nazgul thing.
I imagine that Heren has the same sort of voice.
Eomer is, of course, Scottish. But since the only actual Scot I've heard up close and personal is a middle-aged female, it means that Eomer's voice had to be drawn from television, so on some days, he sounds vaguely like Sean Connery, and on others, he sounds more like Scotty from Star Trek.
Saucie says "aluminium".
Bethberry has the sort of calm and nice voice that could calm a frantic horse. Yes, that's a compliment. Frantic horses are nuts.
Thenamir doesn't talk so much as he booms. Anybody know Lord Beliquose from The Woad to Wuin? His speech (yes, his indoor voice) IS A LOT MORE LIKE THIS.
Kuru is John Rhys-Davies.
Sharku is Christopher Lee.
Everyone else (generally) is a pretty close rendition of my own voice.