PC audio problem?no sound

i have a prob with my PC

while playing black ops game there is NO SOUND but there is SOUND WHEN the GAME STARTING and end movie

and the game was not saving,...object completed but when i close reopens the game it It starts from 1st stage

In game settings >there is no audio device

just now i reistalled ...the game 2 times

please give me a solution

OS: windows7

INTEL i5-2320-3.0ghz

hynix 4gb ram

ATI HD6750 ddr5 1gb........please help me

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