Thanks so much for all your diagnostics, really appreciate the time you've spent on this.
I think the bug lies in the UI - I ran out of time and didn't add an onscreen prompt to press R to start. While it is in the description, it's very missable, I'll make sure to add control prompts in game next time.
The messages you see are Unity trying to start playing audio in HTML5. The web standards prevent things (ads/malware) from randomly playing audio unless the user is interested in them. Only once you click on an HTML element can that element create an audio player, so Unity tries and tries again until it succeeds. I don't know what the benefit is for repeatedly trying over responding to a focus event or something similar.
Thanks again for checking everything technical for me.