In Firefox you can work around this. Click on the padlock next to the URL, then click the Right Arrow, and More Information. Head to the Permissions tab, scroll down and find Autoplay, and switch it to "Allow Audio and Video." Refresh, and you should have sound.

For whatever reason Firefox blocks it, tells you in the console that it blocked it, but fails to present the extra little UI in the site info dropdown that would usually expose the user override. Might be a bug with how this particular game sets up the web audio context or something.