Videos played too fast on YT/firefox

Since a few days, youtube-videos on firefox are played too fast. It happened suddenly, at night I watched a YT video on tv, with hdmi cabel and on the next morning the videos were suddenly played too fast, without any upgrades or other changes in between. What could be the problem?

Opensuse 13.2 64bit KDE 4.14.9

Aha, the responsible for the videos speed and lack of sound was the flash player, I installed a html5 add-on and now (most) videos are played as they should.