Looking in the aircraft folders of the included Asobo, and my addon aircraft, all the sounds are included in a .PCK file, so it looks like the way sounds were used in FSX and P3D is quite different for MSFS2020.
I don't know how you can pack the sound files into a .PCK (unless it is something like a zip file with a PCK extention (.PCK instead of .zip)) so they can be used with the CS 777.