In the last week, I noticed that about 10% of Youtube videos play without audio in all three browsers (IE, Firefox, Chrome). For example, https://www.youtube.com/watch?v=ic4PQ-tn
I re-installed Flash. Update: now it’s worse, and almost none of the videos have sound. Under IE, volume controls disappeared. In a lot of videos, music plays but voices either play extremely faintly (with volume at 100% in video controls, PC playback and on the speakers, I can barely hear them). Almost as if the playback depends on specific codec used by the video authors.
Stranger yet, known good tracks have functional instrumental music but the vocals are completely scrambled (https://www.youtube.com/watch?v=l1qc_zg2