Firstly, do the following:
Right-Click on the game in Steam -> Properties -> Verify Integrity
If this doesn't help, it may be an issue with OpenAL.
To install, search your game download folder for 'oalinst.exe'. Double click on this to install OpenAL, and then re-launch the game.
If for some reason you can't find the file 'oalinst.exe', it can be downloaded from a reputable online source such as download.com