Single channel of audio not working

Hi..I was given a Dimension 2350 desktop with XP Pro that had been used at my workplace. Our IT guy wiped everything from the computer leaving only the operating system. I have had the desktop sitting for months and decided to fire it up. I installed all the waiting Windows updates and there were many. The system seems to work fine but the audio is only working in one channel and is intermittently distorted. I went through a thorough check of cabling and checked the Sound and Audio Devices properties in the Control Panel and everything looks fine. I'm certainly no computer genius and was looking for some advice as to what may be wrong here. Could it be a new sound card is needed, drivers downloaded? I know the speakers function fine. Any help is appreciated !!