It seems that the behaviour of the addVolumeListener callback is called differently from iOS and Android when the volume is set within the library.

On Android, when you set the volume with setVolume, listeners are unregistered:

    @ReactMethod
    public void setVolume(float val, ReadableMap config) {
        unregisterVolumeReceiver();
        ...
        registerVolumeReceiver();
    }

then registered again, when the volume has been updated, so the addVolumeListener callback is not called.

On iOS, there is not such things:

RCT_EXPORT_METHOD(setVolume:(float)val config:(NSDictionary *)config){
    dispatch_sync(dispatch_get_main_queue(), ^{
        id showUI = [config objectForKey:@"showUI"];
        [self showVolumeUI:(showUI != nil && [showUI boolValue])];
        volumeSlider.value = val;
    });
}

So addVolumeListener callback is called when you use setVolume.

I tested to just remove the unregisterVolumeReceiver(); and registerVolumeReceiver(); on Android, and it works like iOS. And that's the behaviour I would like to have.
But now I'm worried if there is something that I didn't see and can lead to memory leek, strange behaviour, hidden infinite loop, or performance issue?

It it's the case, it could be nice to update the iOS part and add this:

RCT_EXPORT_METHOD(setVolume:(float)val config:(NSDictionary *)config){
    __weak RCTSystemSetting *weakSelf = self;
    dispatch_sync(dispatch_get_main_queue(), ^{
        [weakSelf stopObserving];
        ...
        [weakSelf startObserving];
    });
}

Anyway, let me know if I can help and do a PR in either way. Because I think the best thing is to have the same behaviour between platforms right?