Are there cases of shotcrete being sprayed on the underside of metal floor decking for the purpose of sound transfer reduction and dampening between levels? If so, are there special application methods and formulations?

We could not identify any specific cases of shotcreting onto metal decking for sound dampening. However, shotcrete is used in a wide variety of overhead applications, and as long as we can get good bond to the underside of the metal decking, should perform well. Depending on the surface profile of the metal decking, you may want to consider attaching studs to the decking to enhance the overall bond of the concrete. You may also want to consider using lightweight shotcrete since the concrete sounds like it is more for acoustic purposes, than for structural.