I suppose that right now, you have a "programmation" that is "each time you click this button, add 1 to the variable (i don't know your variable's name) or it is when you enter a layer/slide after clicking a button.
To avoid someone clicking the same button and still managing to arrive to the "assessment part", you keep the "add 1 when you click on a button" but you add on the bottom of this event in the condition "If the state of my button is "normal".
In that way, after visiting a button, you change his look (it becomes darker). So it's not the same state. SO if someone clicks again on the same button, since the state will not be the same, the "add +1 to the variable" event will not trigger.
For the audio, after I arrived to the main menu and clicked on "return" on the player, I heard the audio well.