Can I turn sound effects on or off?

Yes: Settings › Learning › Sound effects. You hear a chime for a right answer, a buzz for a miss, a fanfare at 3, 5 and 10 in a row, at the end of a set and when you reach a new level. They start on, quietly: set their volume there, or turn them off there or with the Effects button at the top of a quiz.

They're separate from "Play readings aloud", which plays each word's recording. In the same place you can turn on a short shake of the question when an answer is wrong (never with reduced motion).