Sound has suddenly stopped in Safari (while watching YouTube videos). I have tried the 'command option P R' restart option - to no avail. I've opened iTunes, played a song showing that the internal speakers work fine, adjustable by the F11 and F12 keys. I am running Yosemite 10.10.5 on an iMac 21.5" )late 2009). Has anyone experienced and corrected this OS X malfunction before? Sincerely, PaulRanger2.