I recently bought a Belkin mini DisplayPort Thunderbolt-to-HDMI cable so I could watch videos played on a MacBook Air on my TV. Unfortunately, while video played on the TV, there was no audio. Video played, but there was no sound.
To get audio, I followed the instructions on this page: http://www.justanswer.com/mac-computers/5kqkc-no-sound-youtube-sound-everything-else-i-e.html.
The instructions from the wise Mac Druid were to delete the Macromedia folder: ~/Library/Preferences/Macromedia
Open up the Terminal app and type these two commands:
rm -rf Macromedia/