I’ve done this the derpy DIY way in the past, get a radio transmitter, plug into source with a 3.5mm jack and and just use receivers wherever plugged into anything you like. I paid $5 on ebay for sender and can pick up on phone, car, or dedicated radios within about 15-20m, sounds as good as a normal clear radio station, so probably not audiophile suitable, but as good as normal multi-room fill with untreated spaces will be anyway. If worried about regs, you can be Bluetooth ones for like $15, not sure if there is a limit of pairing multiple at once though, so you many need several senders. If you can, you WILL get best results running a cable I suspect.
Also from the other thread… this is economical if limited rooms.
You can set up groups if you have multiple chromecast audio’s