1. A wearable cardiac defibrillator (WCD) system, comprising: a support structure configured to be worn by a patient; an energy storage module configured to be coupled to the support structure and to store an electrical charge; a discharge circuit configured to be coupled to the energy storage module and configured to discharge the electrical charge through the patient; one or more motion detectors coupled to the support structure, at least one of the motion detectors being coupled to the support structure at such a point as to be near a back of the patient while the support structure is worn by the patient; and a speaker system coupled to the support structure and configured to output a transmitted sound having substantially periodic contents designed to assist a bystander to perform CPR chest compressions on the patient while the support structure is worn by the patient, and in which the performed CPR chest compressions are detected by the one or more motion detectors and the transmitted sound further communicates feedback regarding the detected CPR chest compressions.