Actually, upon closer inspection, in Chrome it starts in a buggy state where the sound is enabled but the speaker icon (at the bottom of the page) is set to mut

Actually, upon closer inspection, in Chrome it starts in a buggy state where the sound is enabled but the speaker icon (at the bottom of the page) is set to mute. In Firefox, the buggy state doesn't occur: it's initially muted and you have to click on the speaker icon to enable the sound.