Use the Volume Mixer in Windows as explained here.
There exists no switch for it as far as I am aware. You could only manually mute each tab by clicking the favicon every time.

Thx but your solution is bad.
Finaly I install opera extension.

  At Opera site: http://www.opera.com/computer/portable
    

This is x64
Where is x86?

Are you on a x86 system? Is your browser 32 bits?
The installer should download the version compatible with your system.

i have x64 OS and i need to download x86. So i need to find computer x86 and download and install on it? it is bad...

bump
leocg help plz

At Opera site: http://www.opera.com/computer/portable

This is x64
Where is x86?

Where i can download opera portable for x86?

maybe like this

\Opera\launcher.exe -enable-tab-audio-muting

There is no setting for this. You can mute Opera using Windows' 'volume mixer'.

Mayby i can change any file \Opera\profile\data\Preferences or like this?

How to disable all audio and sounds in opera?