If you right-click on a .cwp file and go to "Open With>Choose Default program..." you are given a choice of programs.
If you click on (I am checking this in Win 8.1) "More Options" and then, at the bottom, "Look for another app on this PC", you can navigate to C:\Program Files\Cakewalk and set any of the SONARDPR.exe files of any of your installed Sonar versions as the default startup program, and the icon that appears for files with the .cwp extension will be of the default version you choose.
If you create, say, a text file, then change the extension from .txt to .cwp, the icon changes to your default version of SONAR.
If you double-click on the file, your default version opens it with an empty project (!) So Sonar knows nothing of the contents of the file.