You should try the Bowers and Wilkins system in the BMW iX some time. In the USA it’s a $3,400 upgrade compared to their mid-range Harmon Kardon system (which itself is an upgrade from the base system), but it sounds amazing. 30 speakers including one in every head rest, two subwoofers under the floor, and 4 bass shakers in the front seats. 1615 watts total. Sounds better than some home cinema setups I’ve heard.
It’d drain the batteries quickly. Wifi uses a relatively large amount of power, and low-power wifi variants like HaLow aren’t widespread yet. Bluetooth has a low energy mode for low-power devices that works well for this use case. I guess it could have a power cord, but then why use wifi when it could just plug into the phone instead?