Installed:
sudo apt install pulseaudio-module-raop paprefs
Ran paprefs and check the option to use AirPlay speakers locally.
Restarted pulseaudio:
pulseaudio -k
# (may not be necessary
# if yours auto-restarts)
pulseaudio --start
Opened the controls (pavucontrol) and saw the AirPlay speakers.
Main issue is that if I stream to those speakers, whatever I'm playing stops and playback doesn't work.
After a lot of digging, this was the issue: https://askubuntu.com/questions/1029951/wrong-ip-for-airport-express-v1-airplay-in-pulseaudio-raop-ubuntu-18-04
The AirPlay IP displayed in pavucontrol was something like 169.254.xx.xx (really unusual, rather than 192.168.xx.xx).
Based on that link, I did:
sudo apt install avahi-autoipd
Then ran:
sudo avahi-autoipd --force-bind wlan0
which sets up an appropriate address or something (not totally clear on what it's doing).
So I have to leave that running for it to connect properly.