There's a small "settings" button to the left of the URL. Click it, and if it shows Sound already, enable it. Otherwise, click "Site settings" and enable it from that list.
Is there a way to do this in Firefox?
I am having trouble making it work on Firefox. I guess I could add an option to show a little button. Playing audio is allowed, it's the autoplay aspect that isn't.
Most browsers will accept a mouse click or screen tap as user action which allows audio playback. I've used overlays to solve this problem, for example on https://cafegoblin.com (shameless plug for my friend's zine)
I couldn't get it working in Edge, Firefox, or Safari. Eventually got it working in Chrome following the instructions already provided