It should be really great if the home assistant developers will give access to the media/cast icon shown in their default media_player. Because of the mini-media player with more options, it should be great that the creator of that can access the media icon as well. Especially because of the missing favorites on the Sonos side of the media_player, the lack of this icon is even bigger.
See the github topic on the mini-media-player side, the creator is waiting to get his hands on it!