Well, finally I got it working again.
After I installed my OS, I installed an old creative driver and the Alchemy software. The creative Autoupdate then installed a newer driver and it wasn´t working.
Now I cleaned all creative drivers, removed Alchemy and installed only the newest driver.
Tried to run BF2.exe and it stopped immediately with Error 0x000142. *panic mode on*
I removed the dsound.dll and dsound.ini that alchemy had copied in my bf2 folder and after this I was able to start BF2 again. *panic mode off*
This means obviously that the BF2.exe is using the alchemy files in its folder and these apparently need libs from elsewhere themselves, that I had uninstalled.
Still BF2 ran only in stereo, so I reinstalled Alchemy and told it to put it´s files to my bf2 folder again and voila 5.1 Sound in all it´s analog transmitted glory.
Seems like I just should have uninstalled and reinstalled Alchemy after the driver update all the time
And I´m even more confused now about what soundsystem BF2 uses or what is supported by this Alchemy software..
Thanks for help !