Bingo! Thank you. Turning off silent mode and refreshing the page fixed it.

That's very strange though, I wonder if it uses a weird sort of web calling API or something for the audio. I've never had an issue with audio in web pages before, and even the youtube video the page embeds sounds fine. It seems quite poor UX (of Safari? Chrome?) to require turning off silent mode – many people never turn it off so it's not very discoverable, and then forgetting to turn it back on again could be pretty bad.

You are not alone! It’s indeed very bad UX on the part of Apple.

I run a website with music education tools and we had to make a dedicated help page for this particular annoyance:

https://www.soundslice.com/help/en/player/troubleshooting/23...

You're confused why a feature called "silent mode" would make your phone not make sound?

iOS has changed over time, such that silent mode affects notification and interaction sounds but not multimedia - I can still listen to music or watch videos regardless of whether the phone is on silent or not.

"Silent mode" is not the same as "mute". With the former you still expect to hear sound if you're using a function where sound is critical.

There is no other media content that is disabled by silent mode. My mental model for silent mode is that it silences things outside my control - like calls - but not things that I explicitly ask for, like music.