Dell Optiplex 745 Sound Card Problem

Sir I have Optiplex 745 system and its sound card has no audio in its left channel. Inner speaker is working fine but in headphone only one channel is working. I replaced Headphones but still the problem is same. Same problem in front penal and rare panel. I installed windows xp and windows 7 too, but the problem is same. I think its a hardware problem. Is there any hardware expert who could guide me about the fault. And which component should I replace?