I got a used laptop from a computer repair shop

Like other mylotter said, try uninstalling drivers completely then reinstall it. Also, try to find out latest version of drivers for your sound device from internet, it may help also

Hi have you checked the audio driver that you are using. If it is running atleast by the CD it should run good as well as in the internet media player too right. I have doubt about the media player that you are using for both application may not be the same software or player. If different media players please check the advanced settings on both players and control panel too. Also check the your audio hardware's spaecker sorround sound setup that you are using. You may find some difference and easily you can rectify it.

This may be your audio device setting problem. If you have sound driver then first uninstall it and again install properly. I think it will work properly if there is no problem in speaker side.