Instructions: 1- Make sure that you have OpenAL installed (link below). 2- Unzip the four .dlls (wrap_oal, xrSound, OpenAL32 and soft_oal) to the "bin" folder of Call of Chernobyl mod (make a backup first). Start up the game and there should be a new sound device showing up in the sound settings menu, called "DirectSound". 3- Select that, enable EAX and restart the game. Enjoy surround sound.
If necessary, download OpenAL, that renders 3D sound and is compatible with S.T.A.L.K.E.R., from here - Openal.org .