Could be some software process running in the background that's either using a lot
of CPU cycles or generating interrupts so frequently it's interfering with the sound transfer to the speaker jack.
- A tower system, notebook or one of those proprietary systems that don't have the standard tower/desktop arrangement (standard power supply, standard back plane, slots etc)
- Have you been monitoring the computer in any way to make sure processes haven't been accumulating in the start up lists and registry that are using more of the computer's resources than when it was newer? Some consider it an extreme measure, but an occasional reformat and re installation can clear out a lot of extraneous things like browser tool bars, browser add ons and software that came with printers or cameras you haven't used for years.
- Are sounds stored on your hard disk (like tracks ripped from your music CDs) garbled when they are played in Windows Media Player and Media Player is the only application you are running (especially when the browsers are not running)?
- Is the computer equipped with a wire connecting a CD or DVD drive directly to a sound card? If so, do CDs sound garbled when you use the computer to play them?