Try to uninstall and reinstall Flash.

Flash Player uninstall: http://www.adobe.com/go/tn_14157 (this will remove the Firefox Flash plugin and the ActiveX control for IE) --- See also What to do if Firefox won't play any sounds and Fix common audio and video issues

This problem just started this past week. I can see videos but can't hear them on certain sites like YouTube and PlanetPulse at http://www.dailyhoroscope.com/free-horoscopes/videos/planet-pulse-astrology/?feature=dhe-email-tarot-planet-pulse&utm_source=dhe&utm_medium=email&utm_content=dhe-email-tarot-planet-pulse&utm_campaign=promo-code&. I don't have the same problem on Bloomberg.com video or any other URL. When I download videos from YouTube to RealPlayer, I can play them with sound all right. I read your Tips for this problem. I uninstalled Adobe Flash and re-installed it not only using FireFox but also using Explorer. I used the 32 & 64 bit version. I even re-booted my computer but nothing helps. I have Windows 7 and the latest version of FireFox. All my FireFox plugins are up to date too. What can I do? This problem just started last week.

SUCCESS - I now have sound restored to my videos on YouTube and Planet Pulse videos. Here's what I did: I read the info above and uninstalled Flash Player using their uninstaller. I rebooted my PC. I went to Flash Player Help | Find Version at: http://helpx.adobe.com/flash-player/kb/find-version-flash-player.html#main_Find_the_Flash_Player_version_installed_on_your_machine and went step by step through their Support provided. I then went to: http://helpx.adobe.com/flash-player/kb/flash-player-games-video-or.html because I still had no sound with the new installation of Flash Player. I think the single MOST IMPORTANT thing I did was in Step 7: Disable Flash Player hardware acceleration and had to deselect Enable Hardware Acceleration. Then I could hear the sound with my videos again!