The Fujitsu Siemens Amilo Si 1520 contains a Conexant High Definition Audio Chip called CX20549. The notebook comes with a preinstalled Windows XP Home.
I installed the Professional version of Windows XP but the audio driver did not work.
The support hotline told me that this notebook and the audio driver only works with XP Home. Of cause, this is totally wrong, because all versions of Windows XP use exactly the same driver architecture.
After a little bit of research I found the ALB R14 notebook with the same audio chip. There was also a hint to the Microsoft knowledge base article 888111. Unfortunately the hotfix in this article is not available for download. But the driver package for the ALB R14 notebook contains the hotfix.
After installing the hotfix everything was fine. The audio and also the fax modem works. I think the preinstalled version of XP Home is already patched with this hotfix. Too bad that nobody from the support desk can tell.