As I recently reinstalled Vista (as I explained in my previous post), all the drivers for my Vaio laptop had to be reinstalled too. It all went well, except for the sound card. It's a Sigmatel card (High Definition Audio Device).
The problem was that the sound output was OK, except when the headphones were plugged in. In that case no sound was heard, neither from the speakers nor from the headphones.
The solution was not so easy to find, but it's simple enough:
Plug the headphones in
Right-click the speaker icon in the system tray and choose "Playback Devices"
Select the headphones and click "Set Default" button