I think the best audio versions of CoN I've ever heard is the Focus on the Family Radio Theater productions.
They're not just audio-books...they're practically movies without the movie screen. No, scratch that -- they are movies that play on the biggest screen of all: your imagination.
They're just flat out INCREDIBLE. The voice acting is great, the score is wonderful and the sound effects are AMAZING. I cannot stress enough how good these productions are and I highly reccomend them to every fellow Narniac here.
BTW, they haven't only done the Chronicles; they've also done lots of great classics, like Little Women, Anne of Green Gables, The Secret Garden...there's lots of others (and they're all FANTASTIC) , but I can't remember them all.