Adding a play audio on success on a quiz

Are you talking about having an audio play for success on individual quiz questions or when you get to the Quiz Results slide?

If it's for quiz questions, just add the audio clip as object audio on the Success captions.

If it's for the Quiz Result slide, and you're trying to use the On Success event for the entire quiz, that event doesn't fire until just after the END of the Quiz Result timeline.  If you want the audio to play earlier, you will need to set up a Conditional Action that gets executed On Slide Enter of the Quiz Result slide and have the condition look for the value of the System Variable that will tell whether or not the learner has passed the quiz.