Windows Media Player can't play several files in the playlist. To get info about each of the specific problems, click the icon next to each file with a problem.
Sometimes, you might get this error if there's a problem with your sound device, such as a sound card or sound controller. Here are some things you can try, if this is the problem:
Check Windows Update for any available video card updates or checking your PC or video card manufacturer's website for an updated driver.
Make sure you've got a sound device (such as speakers) set up and plugged in correctly. Check the info that came with your device for instructions.