The game installed fine and I decided to play it offline only to find that there are no voices in the game. No GLaDOS, Wheatley or anyone. Is this a glitch? How do I fix it?

In Steam, go to Properties -> Verify integrity of game cache. You might have a broken file somewhere, doing that will re-download anything that's corrupt or missing.

Did you pirate the game?

Did you have your sound off?

I wonder if Valve could do what the creator of Garry's Mod did relating to pirate copies. Anyone else remember that?