Or have the "click to start" text cliclable and start the game with sound. Anyone who wants it muted will make sure to first click the mute symbol and then the ambiguity resolves itself anyway.
[ CLICK TO START ]
[x] Allow sound
I would also argue the MathDoku problem is different. That sounds like a mode confusion type issue, where the user expects a certain level of automation but it has been disabled by the system without adequate feedback.