No audio Output device is installed on Vaio

Hi,
i've a big problem with my laptop vaio vgn-z21wn (http://www.digitec.ch/ProdukteDetail...tikel=156685): No audio output device is installed appear.

I've tried to install the last drivers from the sony and relatek website for windows 7 (prof version x86), but nothing works.

If I go on "device manager" no audio card is present. If i scan for hardware changes, nothing changes.

Please help me, thx

new strange thing...
- after a restart of the system the audio appeared automatically: all was fine
- than i've restarted the computer again, and again the old result "no audio output device is installed"

How is this possible? how can I fix this?
Is a driver or hardware problem?

thx

In the device manager
if I try to insert manually a sound device with a realtek driver the error is: this device cannot start. code 10.

please help