When you initially download the game, the language setting of your Steam client will determine whether (and which) voiceover audio files are downloaded. If you cannot hear sound in the game, you can follow these steps to download the files for any officially-supported language:

1. Uninstall the game.
2. Click the Steam menu, and then select Settings.
3. Select the Interface option.
4. Change the Language you want the Steam store to use.
5. Restart Steam.
6. Re-download the game.

Once you’ve re-downloaded the game, you will have the language files determined by your Steam client.