Pulseaudio lets you turn any device into a speaker over Bluetooth or IP.

Tried this with an android client and experienced substantial latency despite working just fine between 2 computers in the same lan.

Mpd seemed to work better.

Snapchat is good too, and already has Android clients

*snapcast: https://github.com/badaix/snapcast

grr... that was from my phone and autocorrect changed it; too late to edit now.

That's super easy already. You can make the phone act as a USB sound card, or you can use PulseAudio's network transparency (see https://puri.sm/posts/speak-to-me/), or you can broadcast an RTP stream, or you can use Bluetooth audio...