I couldn’t find a simple plug and play solution, so it took several tries to find the right guides, especially because raspbian have changed their Bluetooth/audio setup, so the old guides don’t work.

The solution I found uses a script/daemon that sets up the speaker and waits for Bluetooth connections.

I’ll look for the script…

Edit: found it https://github.com/fdanis-oss/pw_wp_bluetooth_rpi_speaker

I added a systemd service running as root.