LMom,
You're right. It all depends upon the reader. You have to check the cover to see who the reader is. I have no clue who anyone is, but DH recognizes all the actors. My only requirement is that the books have to be unabridged.
DS loved these audio books - not from the site Kriston listed, but through the library or the stores (note, often there are English accents which might be a bit tough to follow for the young ones, depending):
Beverly Cleary, The Mouse and the Motorcycle
Complete Stories of Winnie the Pooh (British accent - no surprise)
The Chronicles of Narnia
A bunch of those tapes that come with books in the children's area of Borders and Barnes and Noble - Peter Pan, Dr. Seuss, Monsters, Inc., a bunch of others (For the most part, they're read well and clearly in an entertaining manner. Our only problem was that they were so short, and therefore not cost effective.)
Bill Bryson, A Short History of Nearly Everything (also a British accent, but excellent reader. Note: the tapes are unabridged, the cds are revised)
Stephen Squyres, The Mars Rover Program (quite engaging, some minor swear words)
Dud:
Stephen Hawking, A Brief History of Time (don't bother; poorly read - better off reading it aloud yourself, which DS enjoyed -- if you do that get The Complete Illustrated Brief History of Time - great illustrations)
Then there are audio books read by Broadway actors which are awesome. Geronimo Stilton on CD is what made DS5 hooked on his books. 