Trigger to show next button - Trigger tied to end of audio?

I want to have the next button show up on the slides only after the audio gets done playing on the slide.  I've tried to just add the next button on the timeline when the audio finishes, but it's not coming in exactly right.  There must be a more precise way.  Is there a way to show a layer when timeline ends on audio?  I'm only seeing the possibility of showing layer when timeline ends on an object.  What am I missing?  Anyone have a suggestion? 

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