How to stream audio to multiple AirPlay devices
I must sound to some others on this site like I'm paid to advertise* Rogue Amoeba ;-))
Basically, if it's Mac audio routing, this is the go-to company.
This is one of theirs I've never needed so never tried, but their stuff is usually absolutely rock solid. Have a look at Airfoil
* I'm not, of course, but I wouldn't turn them down if they offered ;)