Hello again HansNorge,
Thank you for responding to my post requesting additional information. I had a Moderator remove your serial number as it is not a number that you should make public. As for your product number took me a bit, but I figured it out your product number is KM030EA and you have a HP Pavilion dv9750eo Entertainment Notebook PC. You may want to document this for any future help you may need on this computer.
Now the only drivers available for this computer are for Windows Vista and that is most likely why you have run into issues with the audio not working correctly when you have upgraded to Windows 7. If you could I would like you to download and install the Conexant High-Definition Audio Driver and see if it will enable your sound to work again.
Thank you for posting on the HP Forums. Have a great day!