Windows Media Player Glitch?

Well, I have windows 7 and for some reason some videos (MP4 Format) dont work on WMP, they lag, the sound doesn't work and in its place is some kind of robotic screeching sound. However, on windows 7 when you open folders you can 'preview' things, wether it be doents, pictures or videos - and when I play them on there they work fine. Any idea why this is happening? They used to work fine on WMP, but reacently not.