Вот пример установки громкости на 50%:
Float32 volume = 0.5;
UInt32 size = sizeof(Float32);
AudioObjectPropertyAddress address = {
kAudioDevicePropertyVolumeScalar,
kAudioDevicePropertyScopeOutput,
1 // use values 1 and 2 here, 0 (master) does not seem to work
};
OSStatus err;
err = AudioObjectSetPropertyData(device, &address, 0, NULL, size, &volume);