The thing about Pratchett's talking books—audiobooks, I mean—is that you really need a narrator who gets the footnotes. That's non-negotiable. For a beginner, I'd honestly say skip the early Rincewind stuff and go straight for the City Watch arc. 'Guards! Guards!' read by Nigel Planer is where I started my kid, and she was hooked by the third chapter. Planer's Vimes is grumpy perfection, and he handles the shift from silly to serious so smoothly you don't even notice the philosophy creeping in.
Some people swear by the newer ones with Stephen Briggs, and his performance in 'Going Postal' is stellar, but Briggs's tone is drier. For a total newcomer, that might feel a bit slower. The Death novels are fantastic, but 'Mort' as an audiobook? The whimsy can sound a bit twee if you're not already bought in. My two cents: start with the Watch, get a feel for the world, then branch out. The Moist von Lipwig books are a great second stop—they're practically heist movies.