I heard a similar complaint and it turned out they had downloaded some video off the net and it was a bum video. Another time the video had only 5.1 sound and your PC often only does stereo.
It's amazing folk download video and then think something is wrong with their PC. Good news for the PC repair industry.
Got sound on my computer but if I play a video I get sound but no speech sounds.
I am currently running win 7