No video sound in CS6 Extended

Hello community!

First of all sorry for my bad english!

My problem:

After working with CS6 for many month only for image editing, I started to try editing video clips. But neither when playing videos or editing I get the sound.
This is the case with all videos. If I play it in the Windows Media Player or with the VLC media player, then I have the sound.
What could be the problem?
The switching possibilities with the loudspeaker icon, I know of course.

Thank you in advance!