I have no sound when I play You Tube videos or iTune songs.

I have no sound coming through my speakers on both YouTube videos and my iTunes library songs, on playback.

Previously, in the past year, when I could not hear any sound on YouTube video playback, I saw an answer in another forum which addressed this problem, tried it and I got my sound back! What I did was double-clicked on my iMac hard drive icon in Finder, opened up Applications, then right-clicked on Safari, and then clicked on "Get Info." I noticed that the checkbox "Open Using Rosetta" was checked, and the solution was to uncheck it (then I restarted my computer) and then attempted to play a YT video and I got my sound back!

This time, I thought I had the same problem, but the Rosetta block was still unchecked, yet I can't get any sound to come through my attached speakers, so I'm wondering what the problem and solution are? Can anyone help? I would appreciate it. The first time it happened, I downloaded Firefox browser, and WAS able to hear sound at least that way on YT videos.

Thanks in advance for any help anyone can provide!