incorporated speakers in monitor not detected

Hi,

I have a Toshiba Tekbright Monitor that was working very well in my old computer. I bought a Dell XPS 920 I7 12G RAM 2T with a Windows 8.1, the incorporated speakers are not detected, even after I updated the Nvidia drivers. 

If I plug headers, I have sound.

If I restart and press F1, the audio test there gives me sound too.

So the monitor is fine, just some conflict between speakers and Windows/Nvidia.

Any ideas?

Thank you in advance,

Mireia