I want to use a small audio-interface when traveling.
Therefore, I bought the Roland UA-22.
If I connect it to my computer, Sonar (X1 Studio) recognizes the interface as midi-source, but not as audio-interface.
Do you have an idea what could be wrong?

Did you follow the installation instructions exactly?  Sometimes the sequence of events is very important (For example, connecting a device before it asks you to can mess things up).
 
Make sure you have the latest and correct version of the software (from Roland's website). 
 
Then I would:
Uninstall the driver software.  Reboot.  Reinstall (follow the given instructions), using "Run as Administrator.
 
If you're still having a problem, change USB ports.  For the interface, best to use only USB2 (and not USB3).

 
 
Right  DO what Jon D said to do ... plus double check your settings in preferances 
 
 
also ... if it still doesn't recognize the Roland interface - then politely hold the interface up to your computer monitor and say "Do you see it now" 
 
if it still doesn't recognize it then tap on the monitor and yell "Hello -- McFly!! Anybody Home? Think McFly THINK!" as you hold it up to the monitor. 
 
Both of those methods have worked for me before in emergency situations - especially with Step Children 

 
 
Right  DO what Jon D said to do ... plus double check your settings in preferances 
 
 
also ... if it still doesn't recognize the Roland interface - then politely hold the interface up to your computer monitor and say "Do you see it now" 
 
if it still doesn't recognize it then tap on the monitor and yell "Hello -- McFly!! Anybody Home? Think McFly THINK!" as you hold it up to the monitor. 
 
Both of those methods have worked for me before in emergency situations - especially with Step Children 

Sonar occasionally (really not often) ignores my UA-25. If you loaded drivers properly, then restart computer several times and check audio devices in Prefernces.  

Thanks for your answers. You were right, it was the drivers causing the problem.
After de- and reinstalling them properly, the UA-22 was recognized by Sonar X1!

Excellent, great to hear!  It would be a good idea to edit your subject title and add "SOLVED".