Thanks for the hint.
I did some experimentation with http://xoscope.sourceforge.net/
and the soundcard. But it had some severe limitations:
- limited voltage range, unless you add some voltage dividers and switches
- AC coupling, unless you know which capacitor to remove from the soundcard
- limited sampling frequency of usually 44kHz
So, I bought a http://www.seeedstudio.com/depot/backorder-dso-nano-pocket-size-digital-storage-oscilloscope-p-512.html?cPath=11
Some people complain, it's not a professional device. But for me, it does the job really well. Finally I could dispose my ancient CRT ocilloscope.