Sorry. You're right. It is a sound.ini file, not a sound.cfg file, but even so, there is still no sound.cfg (or a sound.ini) file in the Captain_Sim\777-II\sound folder.

Have you checked in the SimObjects\Airplanes\CS_B757-200-III_PW2037\sound folder (or any other CS 757-III sound folder), because there is a sound.cfg in them. Some will redirect the sound to the another CS_B757-III sound folder which will have all the necessary .wav files in it.