Jean, I am pretty picky about the narrators. My favorite narrator is Grover Gardner, followed by Ray Porter. Gardner is overall excellent and Porter is really, really good with the books that have characters who are fast with the quips/snide remarks/smarty-pants comments and such. Derrick Perkins does a good job with my ancient history non-fiction picks. Carlton Clifton does a lot of interesting history, but as good as his readings are, they come with music and sound effects which, for me, seems a bit much, as in annoying and distracting. A few of mine are group narrations, which as far as the samples go seem good. The Perdita Weeks did a great job of reading Circe, with a breathy, seductive voice that seems to fit the character very well. She is the only female narrator I like so far.
One narrator that I don't care for, sadly, is Wil Wheaton. You may recall him in Star Trek: Next Generation. Will is a friend of John Scalzi's and does the narration for his books. I find his readings kind of flat. I remember thinking his TV persona came off like he never knew quite what to do with himself. His readings give me that feeling too.
Some of my library are multiple character narrations and seem good as far as the samples show. I believe some of them are old live radio performances. Generally, the listening samples are enough for me to get an idea whether or not I will like the book or not. Having said all of that, I have to agree with you about listening to some of the books that I have already read. They often don't fit with my internal reading voice.
Anyway, I am getting quite a collection of audio-books and have only actually listened to less than a dozen so far, and most of them have been library borrows.