No sound from videos

Strangeness abounds! I get no sound from videos on youtube or from the various news sites. Good video, no audio. This is true in Safari and Firefox. When I go to a radio station's "Listen Live," the sound is loud and clear. This station's Listen Live launches iTunes and the audio comes thru iTunes. Another station is reached by the browser and the audio comes in thru streamtheworld; that plays fine. The output Sound in System Preferences is and had been set properly. I repaired permissions and that was strange, too. It took about five minutes to repair permissions and the system showed no permissions needed repair. In seven years of Apple, I've never seen no permissions get repaired.