That sounds like another application, or Windows itself, has taken over the device driver and not released it back to Sonar. Sometimes there's another reason, which is that another application or Windows has changed the sample rate to one other than SOnar is set to.
Do you have Window's sounds enabled? If so, turn them all off.
If you have anything open that might use the sound driver - media player, web browser etc., close them before launching Sonar.
There's a setting on Sonar preferences to "share device drivers with other applications". If it's ticked, try unticking it. That should mean Sonar refuses to hand the driver over to anything else. If it isn't ticked, try ticking it and see if that makes a difference.