HELP!!! Please and Thank you.

1. Set the Initial state of the icon to Disabled.
2. Trigger the icons to change to Normal when the media (aka the audio) completes.
3. As already programmed, clicking each icon shows the corresponding layer. (Be careful about what shows when viewing a layer. FYI, you can hide any base object by deselecting its eye icon in the layer's timeline.) 
4. I don't think it's a good idea to simply show a layer when "all layers are viewed." The user should be in control of when that final layer appears.
5. Give each icon a Visited state. They will automatically change to Visited when clicked. Have another button on the base with its Initial state set to Hidden. Trigger it to change to Normal when the state of all the icons = Visited. If the "final layer" provides additional info, have that extra base button show the final layer. If there's no extra info to show, that button could be your custom Next button.

This post is about controlling the Player's Next button in different situations, including requiring the user to view layers. It still might help:

• TIP: Controlling the NEXT Button 101 | Articulate - Community 

This might help, too: 

• TIP: Making icons easier to program and easier to click | Articulate - Community