I brought up an old project that has 2 Sound Center tracks. I can't hear them and attempts to reload the sounds have failed. Other tracks are fine. I have re-installed Sound Center (and MC One Touch) and re-scanned to no avail. 
I am a bit rusty with MC having laid off for a few years.
 
And what is the difference between Studio Instruments and Sound Center? Both seem to be add-ons to Cakewalk that aren't really integrated and the documentation is vague.