When did this start happening?
Click Start > in the Search programs and files text field type view reliability history > click view reliability history > From the date it started happening this will show things that have happened on your windows 7 computer. Blue circles with ! indicate software/hardware install/update, red circle with x indicate errors.
Also see if your computer needs to be cleaned of dust. Follow proper computer cleaning procedures to do so (google is your friend).
Scan your computer for infections, viruses, malware, etc. FYI I have found that some virus protection do not always detect an infection.
Does this happen with videos just online? Can you play a downloaded video with out loss of audio?