Going a bit wild here...
Cheap double-din car radio, e.g.:
Gets you FM radio for when you want it, and any content off your phone or SD card when you don't, hands free calling so you can answer the phone when your hands are covered in schmoo, 4x45W output, a remote control, RCA out if you want to connect to an external amp.
Some cheap speakers, fitted into your walls (true infinite baffle, almost...): https://www.ebay.co.uk/itm/NEW-Cheap-Budget-700-Watt-6-x-9-Inch-Car-speakers-4-Way-pair-350-Watt-Each/142354731945?epid=7019561990&hash=item212500d3a9%3Ag%3AYEEAAOSwSypY95C8&LH_ItemCondition=1000
Combine with an old PC PSU for the 12V supply and you are away.
Stereo was the first one I came across, there are likely better ones out there.