The embedded objects in quiz slides, those who have no separate timelines, are different from normal interactive objects. There is no real difference between normal buttons and shape buttons as you seem to tell at all, just between embedded buttons that have specific functionality and the normal buttons. There is no way to replace a quiz button by a shape button at this moment, but that wouldn't change the way they act.
If you want audio, you can attach it to the feedback captions (Success, Failure) or put them in actions that are triggered by the Submit button: the Success and Last Attempt actions. More details: