Audio must complete before clicking button

There are a couple of ways you could go about this - without seeing your slide, but assuming you have multiple buttons. What I would first do is either place a blocker over the slide or buttons (e.g., a rectangle shape and then format it to have no outline and 99% transparency) and have the shape end/exit the slide just before the initial audio ends. 

Then, assuming there are multiple buttons, I would select them all, right-click, button set, and assign a button set. This makes it so only one is clickable at a time and would prevent overlapping audio. 

Let me know if this helps :) 

This post might provide some guidance: TIP: Controlling the NEXT Button 101 | Articulate - Community 

It includes a file that demonstrates and explains various ways to control navigation.

you could animate in the buttons after the audio completes