Okay, now I see what you’re trying to understand. It is hard to visualize what Home Assistant is (it’s everything! jk) without actually installing it first.
You can use any Amazon Fire tablet and browser (I recommend Fully Kiosk Browser) to load the Home Assistant dashboard (which is a fancy webpage, like http://hassio.local:8123) running off a Raspberry Pi with Home Assistant installed (this is the HA server). There is an official iOS app that works pretty well on iPhone. Don’t know if it displays well on an iPad. The HA demo here https://home-assistant.io/demo/ gives you a good idea of what it will look like on the Fire tablet, or any device that runs a supported web browser.
- Do I have a fire tablet at each location?
You can use any device that runs a web browser, but Fire tablets work well with Home Assistant.
- Do you have a screenshot of the HA audio interface? Or do I need to write my own? I cannot seem to find a screen shot.
I think this is the part you will run into trouble, because Home Assistant isn’t focused on audio. You can add a media player component, like the Spotify player, and that will give you basic functionality like play, pause, next track. See screenshot of the Spotify player on the dashboard here:
The media player shows up as a ‘card’ in the UI. You can click on the ‘three dots’ and you get some additional options, but that’s it for the Spotify component. You can’t change too much unless you write your own custom card, which requires some web development skill.
You can’t browse through playlists with the current Spotify component. Pandora is another component you have to add, and that component looks more difficult to setup as it is using another open-source program called Pianobar (https://home-assistant.io/components/media_player.pandora/)
- Do I have a volume control in addition to the fire tablets?
The Spotify volume control you see in the screenshot above works pretty well, because Spotify’s API lets you set volume control easily. For Pandora and Audible, I don’t know.
Basically, I think Home Assistant won’t be able to do what you want without some major customization
I’ll give you something else to research…there is another platform called Volumio that has a slick web UI to browse your local files from a NAS, and a Spotify plugin that does let you browse through playlists. Unfortunately you cannot use Chromecast Audios or Amazon Echos to play music through Volumio–it will only play through the Raspberry Pi (where Volumio server is installed) or through the USB DAC attached to it. You could potentially install Snapcast server on your Volumio server, and then put Raspberry Pis w/ Snapcast in each of the rooms and connect each to a set of speakers…but you can imagine how messy that gets. I wouldn’t do it unless you have a good understanding of Linux and installing packages on your own.
As of early December 2017, Spotify supports Amazon Echo’s multi-room audio feature. It takes a long time to add support to Amazon Echo multi-room, so I think it only works with Spotify, Pandora, and Amazon Music.