I am trying to imagine what "running at half speed" would be like, and have no clear idea what you are talking about.
Are you saying that the music played by your other audio applications is playing slowly? Is it exactly normal speed/2 or is it some other fractional multiple of normal speed? Is the pitch altered as well? Maybe a mismatch between the sample rate set in your audio interface and the one used by the applications? Are the audio applications and Sonar using the same audio interface?
Check the Sound Devices menu in control panel and see if changing the sampling rate for the appropriate audio device to 44.1K fixes the problem. If Sonar is playing normally, then likely your audio interface is matching the project sample rate, but the other applications do not follow the audio driver SR.