In Google's event demo, Google Assistant was used to bring up photos from the past, and can also play your music when you ask it to play, choosing your favorite default music app. (ibtimes.com)
In that partnership, Samsung's Galaxy S8 and the Galaxy S8 + phones came pre-installed with the Google Play Music service as the default music app, and Samsung users were able to test the new daily recommendation service. (fortune.com)
You can only change from volume from inside the default music app. (goodereader.com)