in src/ios/CDVNotification.m [121:132]
static void playBeep(int count) {
SystemSoundID completeSound;
NSInteger cbDataCount = count;
NSURL* audioPath = [[NSBundle mainBundle] URLForResource:@"CDVNotification.bundle/beep" withExtension:@"wav"];
#if __has_feature(objc_arc)
AudioServicesCreateSystemSoundID((__bridge CFURLRef)audioPath, &completeSound);
#else
AudioServicesCreateSystemSoundID((CFURLRef)audioPath, &completeSound);
#endif
AudioServicesAddSystemSoundCompletion(completeSound, NULL, NULL, soundCompletionCallback, (void*)(cbDataCount-1));
AudioServicesPlaySystemSound(completeSound);
}