Thanks, ScruffR. I got it to play an 880 Hz sound, although I did need to reset the Photon before it would talk to the cloud again, I think because the system mode was manual.
I was trying to see what this – https://go.particle.io/shared_apps/5aef6f695286fce9cb00035e (from your post on Audio, SD cards and speakers?) was all about, but the link just brings me to my apps in the web ide.