There is a 'sound on' setting in Settings. However if you want it on during duels (so you don't miss when your opponent guesses or when you find an opponent) but off the rest of the time (so you can e.g. listen to music when you play classic games), you have to keep switching it on and off and it is easy to forget to turn it on for duels.

It'd be really nice to be able to have a setting to say that you just want sound on for duels.