The icon is associated with the "CakewalkMIDIFile" key and can be found at:
HKEY_CLASSES_ROOT\CakewalkMIDIFile\DefaultIcon That key needs to be modified to point to the dll file beginning with "TTSRES" in your preferred program directory. Using Windows Explorer, the default location (for the Producer version) is "C:\Program Files\Cakewalk\SONAR X3 Producer\TTSRES110.dll"
The registration key listed above needs to point to the specific dll for your X3 installation (it may be a different name), so verify in Windows Explorer when you change that key value.