Stereo Mix Not Working

Sorry, you give no info at all needed for problem solving, especially because Stereo Mix is a feature of your soundcard, not that of SONAR/CWbBL.

This is a user forum. If you wish to communicate with BandLab directly, here is their support information http://forum.cakewalk.com/FindPost/3775869
 
Without knowing any more I can only suggest reviewing your hardware set up https://www.cakewalk.com/Support/Knowledge-Base/Audio-Hardware-Setup-Guide use SONAR Platinum for the Product drop down.
 

i HAVE recently installed software for a Focusrite interface, which may be affecting the way Cakewalk by Bandlab receives my computer's Stereo Mix?
 

If your driver mode in SONAR is WASAPI or WDM and you're seeing both Focusrite and soundcard I/O ports in Audio Devices, you may have a conflict between Playback and Record Timing Masters that's causing distortion. Or possibly you have input echo enabled on the track that's recording Stereo mix, and you're getting a feedback loop because 'Stereo Mix' is the output of the soundcard, and you're echoing the output back to the input.
 
I recommend you set the Driver Mode in CbB to ASIO so it sees only the Focusrite I/O, and letting Windows and multimedia apps use the soundcard to prevent conflicts. Then, if you need to record output of the soundcard into SONAR, do it via patch cable from the output of the soundcard to input of the Focusrite or use some generic multimedia app to make the recording from Stereo Mix, independent of CbB.