Not to be confused with The Prototype.
The Game use native 5.1 there is no stereo Downmix.
So in a Stereo setup you don't have the Center and Surround Speakers so the volume is very low except for Cutscenes (they are in Stereo).
Use an alternative Sound Driver like Razor Surround. It's simulating a Surround effect while using the Full Speaker Configuration.
If you have a Asus Xonar Card open the Xonar Control Center in the Mainmenu select 8 at Audio Channel and 2 at Analog out. Now The Asus driver makes the Downmix from any Surround Source.
I have tested both variants and use the Xonar solution because i have these card so no need to install any other thirdparty Software.
Prototype on an Nvidia graphics card.
Prototype on an AMD/ATI graphics card.
Use DirectInput FPS Fix.
See Sound problems (cracking, delays, missing music/effects, etc.)
cmd /C start "steam" /affinity 0xff "C:\Program Files (x86)\Steam\steam.exe"
Enter your computer's BIOS and turn off the hyperthreading CPU feature or turn off all but four cores. The specific procedure depends on your motherboard, thus you have to consult the manual of your motherboard if you're unsure how to do this.