LOL! It was off. Now I feel ashamed. Perhaps a little UX improvement would be to remove this switch and instead add option "None" to the "Sounds" select-box above. I obviously checked the settings, but as soon as I spotted "Sounds: Default", I stopped looking and didn't even notice there was another switch down below. My bad, but perhaps this would prevent other people from falling down the rabbit hole 
BTW: I didn't disable the sound, don't know why it went off. But like I said, I saw some unreachable files in the console earlier so there might be some connection there...
Anyway, problem solved. Thank you.
It used to play a sound on each move. This no longer happens. I'm not aware of changing any settings or browser configuration. Sound works fine on other websites.
Environment: Debian GNU/Linux 10, Google Chrome 89.0.4389.82 (I also tried Chrome 88 and Firefox ESR from the current stable without any extensions).
I believe this is a technical problem on your side, but surprisingly I coudln't find any other reports in this forum. It might be possible that it affects only users in some regions (because they get directed to a certain CDN or something).
Earlier, I tried to look into the console and saw a few failed requests to some sound assets, but this no longer happens -- and yet, the sound is still not working. I have no idea what's going on.
It's been a few days since it stopped working.