AirPlay is as about as good as it gets I think. It’s been thoroughly reverse engineered so there’s lots of open source tools for it and it’s cheap to add speakers since used AirPort Expresses are still supported AirPlay targets.
That makes it easier, but no. Many implementations of both servers and clients (both closed and FOSS) exist, for example AirMusic adds AirPlay casting support to Android and I’ve heard of people using rPi’s as receivers. The protocol is entirely local, there’s no remote server component at all.