I read that file, noticed a reference to /static that is referring to an html folder.
I added my doorbell.mp3 to the {openhab home}/conf/html folder and that did the trick.
I can now play a doorbell notification from the Sonos using this command:
sendCommand(Sonos_Living_NotificationSound, "http://<oh server ip>/static/doorbell.mp3")
Fwiw, .wav files work as well.