Want to make a Raspberry Pi powered wifi audio system, looking for advice on my shopping list

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Like the title says, I’d like to run an RPi powered headless WiFi audio player integrated into some custom made speakers. Here’s the list of parts I have figured out so far:

Here are my main questions:

  • What’s the best way to connect the speakers? What kind of wires are best for this application? I’m thinking maybe I’ll integrate the server into one of the speakers and wire up the second speaker to act as a satellite, just to keep the footprint down as much as possible. I may also wire up both speakers into a single unit if I’m not as concerned about stereo sound.

  • What else would I need to supply a line-in input to this setup? I want to be able to send audio from a digital piano to the speakers. I could always get a 3.5mm jack and wire it up, but is there anywhere for it to connect to on the RPi, audio bonnet or amp? Is there a better audio interface for the RPi that would allow this?
    (EDIT: I had a thought about this - failing everything else I could add a physical switch to the system that would sit between the audio bonnet and the amp and switch between the internal audio bonnet and the external line-in. I’m only going to use the piano in physical proximity to the speakers, so that’s a good solution. I would rather not need to log into the wifi system just to use the piano. I’d just need to know if the amp can handle switching from digital to analog signals by itself, because I’d be sending digital from the audio bonnet and analog from the piano.)

  • What kind of power supply would be best for this? I could get a 5V DC PSU. That should provide enough for the driver and the RPi. Alternatively I could get a 12V DC PSU and add a step-down regulator to power the RPi. Any thoughts on what would be a better way to go? Also the 5V PSU is currently not in stock, and it’s not clear how long that will take to arrive. Maybe the answer is obviously the 5V but I’m interested to know.

Now here’s extra thoughts in case you’re interested:

Ideally I’d like to be able to set it up across multiple rooms, so if I build more units I can extend the network of players to play in sync or independently. That last part is probably a software question, I’ve found a few OS choices I can use:

I’d like to build the entire system including speakers, so I can print my own funky speaker boxes, spherical, nautilus, death star, 360, whatever.