Youtube - audio is out

Youtube - I cannot hear audio from "Youtube" and similars

My browser is I.E. 8  32bits , Windows7 at 64 bit, flash player last edition, AV Avira, firewall Comodo.

Now on "youtube" the audio is not possible to hear while, for example, on msn video is all in order. also the audio of videos downloaded in my pc is in order.

http://http://www.bing.com/videos/watch/video/365-days-of-beard/208vee4f

I ask for your help to solve the problem.

p.s. I have tried to uninstall Flash player and reinstall disabling antivirus and firewall but without any solution.