I don't have the game but this is apparently a known bug. I hope this info might help you.
STALKER Shadow of Chernobyl Bug /no sound/ - YouTube
http://www.youtube.com/watch?v=-rSwQ-k35-w
S.T.A.L.K.E.R. surround sound fix - Guru3D.com Forums
http://forums.guru3d.com/showthread.php?t=218787
STALKER No surround sound? Possible fix - Steam Users' Forums
http://forums.steampowered.com/forums/s ... ?t=1229614
This info might also help.
http://www.tweakguides.com/STALKER_6.html
Sound
SFX, Music Volume: These sliders control the volume level of the Special Effects (SFX), and any music in game, such as that played in the main menu. Setting these sliders to the far left does not turn remove music or sound effects, so it has no impact on performance. See the Advanced Tweaking section if you wish to actually disable the loading of all sounds, e.g. to troubleshoot a sound problem.
EAX: You can enable Environmental Audio (EAX) on sound cards which support this functionality - primarily Creative SoundBlaster/Audigy/X-Fi cards. When enabled, sound effects will become more realistic in 3D space, but it may also reduce performance slightly and cause audio glitches on some systems.
If you are having audio problems, first make sure you're running the latest audio driver for your sound solution. Then you may want to try switching the audio renderer, either to see if performance and/or functionality improves, or to see if it resolves your audio problems. Use the -dsound command line switch covered in the Advanced Tweaking section. This forces STALKER to use the DirectSound audio renderer instead of OpenAL audio. Note that Windows Vista or 7 users who force DirectSound will automatically be switched to software audio mode and will lose EAX effects, though this may resolve audio problems - see the Sound section of the relevant TweakGuides Tweaking Companion for more details.
If that doesn't work, or you don't want to use DirectSound, for further troubleshooting first make sure your OpenAL version is the latest by checking here - download and compare the file versions of the existing OpenAL32.dll and wrap_oal.dll files under your \Program Files\THQ\S.T.A.L.K.E.R. - Shadow of Chernobyl\bin directory. Some people claim that using this particular OpenAL32.dll file in place of the one in the above game directory resolves issues with EAX and 5.1 surround not working, so perhaps give that a try (backup the original OpenAL32.dll file first). Finally, you can experiment with the snd_ variables, as well as the -nosound switch, covered in the Advanced Tweaking section to see if sound is the source of the problem. If nothing else works, then either a patch from the game's developers, or audio driver updates, or both, may be the only solution.