On Windows 7, in YouTube web client (in latest Chrome) - videos play, but without sound. IIRC, they used to play with sound some days earlier on the same machine. Pretty sure I did not change any config opts in between - had no reason to.
Yet, at the same time (now, when YT has no sound), VLC player works fine, has sound.
Had similar problem earlier on another Windows 7 machine of sound getting disabled intermittently. Tried a lot of things in Control Panel / Sound option, but mostly did not work then, nor now.