I priced out a Sonos system for multi room audio and it was over $6k USD. Decided to build my own system with a combination of:

- Sony audio receivers for each room

- Chromecasts

- HDMI to RCA adapters

- Harmony Pro IR controller

- Raspberry PI with Home Assistant

I set up the Chromecasts with audio groups for the whole house and different areas. I configured the Home Assistant to control power for the receivers through the IR controller when they are cast to. (You could probably skip the IR setup and adapters with more expensive HDMI receivers, the stereo receivers were only $150 each)

Anyone on the WiFi can easily cast audio from any music app to any room, area or the whole house without having to install or configure anything. Works with Android and iOS. Whole set up cost less than $2k.