You'll find it around the 1:50 mark. It's only a few words, but it's... well it's unsettling.
You should also stick around for Cumberbatch's unprocessed Smaug voice (around 2:30), which is genuinely impressive. Oh, and you'll want to hear him pronounce "penguin," also. Because he's unaccountably bad at it. Bit starts around 3:20.