You need to specify your sound card. Try visiting the manufacturer's website. If they have any WDM drivers available, they will probably have them there to download.
You should have been able to tell if Sonar is taking advantage of WDM drivers when you first started. It should have said that it needed to perform a test on your hardware. Then a box would have popped up saying something about WDM. If that didn't happen, then you don't have a WDM driver for your soundcard installed.
Also, make sure that "Always use MME interface..." is NOT checked under Options/Audio/Advanced in Sonar.
I was having problems with my MOTU in Sonar, and I downloaded the latest WDM driver and it made a world of difference.