Controlling the next button to display after all layers / audio is complete

I can't look at your file at the moment. But I can point you to this post:  TIP: Controlling the NEXT Button 101 | Articulate - Community 

The file attached to that discussion demonstrates multiple ways to control the Next button—including how to ensure the user completes every layer on a slide. 

This is how I've done it in your file:

1. Disabled the triggers on the base layer that flip the Layer[1/2/3]Variable to true, moved them to individual layers, and set them to trigger when the timeline ends on each layer.
2. On the base layer, set up 3 new triggers, each changing the state of the Next button to normal when Layer[1/2/3]Variables change, and if each of Layer[1/2/3]Variables is true.

That worked.

Additional tip: Instead of hiding the next button from the beginning, I think it'd be better to keep it disabled so that the user knows it is there and they need to do something to enable it.

SonjaGoodwin​ : The variable placement and triggers described by ShamsShimul is what you'd find in the demo file attached to the post I linked to. 

I also prefer disabling the Next button instead of hiding it. When Next is hidden, the Prev button moves into its place. Someone might click there without seeing it's the Prev button, and then get confused about where they are. 

Here are a few other notes:

The trigger that hides the Next button runs when the timeline of the slide starts. That will happen every time the user visits the slide, even if the Slide Properties are set to "Resume saved state." So if someone returned to the slide, Next would be disabled and they'd be stuck there.

Thus, you need to add conditions to that. Here's how that would look:

Be sure to connect the conditions with "or," because you do want the Next button to be unavailable if any of the variables = False.

 

Variables maintain their value until a trigger changes them. So if you use this interaction on another slide with the same variables, you need to reset them to False when the timeline starts. Or just use unique variables. 

 

There are rectangles covering the buttons. Those rectangles are hidden when the timeline ends. That will prevent those using a mouse from clicking the buttons. However, that won't prevent someone from accessing the buttons with a keyboard. So instead of covering them with rectangles, those buttons should have an Initial State of Disabled. Then trigger them to Normal when the timeline ends.