You need to update the audio drivers -
1. Download the latest http://www.blueripplesound.com/get-update2
2. Uninstall the Rapture3D drivers if old.
3. Install new Rapture 3D drivers version 2.6.5
4. (optional) Set system environment variable ALLOW_RESTRICTED_RAPTURE3D = 1
5. Restart Steam (if you did step 4)
6. In game, change Sound from "Software" to "Rapture 3D", and restart game.
To see the result, check out this uploader instruction.