Lots of code reused from demo-proximity-voice, thanks!
Either generate a self-signed certificate or put the server behind a reverse proxy as voice media can only be used over secure connections:
openssl genrsa -out key.pem
openssl req -new -key key.pem -out csr.pem
openssl x509 -req -days 9999 -in csr.pem -signkey key.pem -out cert.pem
rm csr.pem
Install dependencies with npm install and run with npm start. Open https://127.0.0.1:3000, https://yourexternalip:3000, or https://yourlanip:3000 in a supporting browser on multiple devices.
Edit the userscript to reflect your servers address and port (the port for https is 443)