Occasionally the sound effects or music will drop out on iOS.
I can't identify the exact cause or reliably reproduce an exact set of steps, I've been trying, but no luck so far. There's nothing in particular that seems to cause it, it just stops in the middle of a battle and doesn't come back.
I have noticed that if something interrupts the game, like if the device receives a phone call, then on return the game will "catch up" will all of the sound effects, playing maybe 10 sound effects on top of each other, and then the sound effects will return for a few minutes and then break again.
I am running the newest version available from the App Store. I am on iOS 13.6 (the newest available version when this ticket was created) I have an iPad Pro 12.9 inch (2nd Generation) The device's silent setting in Control Center is off.