Layers & Audio

I have come across a strange glitch regarding Layers and Audio.

I have built a step through interaction using layers to show the next part of the interface.

Each layer has an audio track explaining whats going on onscreen. When the hotspot is clicked the next layer is shown.

The problem I have hit, is that if the audio hasn't finished on the previous layer it will play alongside the current audio on the layer. I have fixed this by using Pause media of the previous layer. Though that has been quite labour intensive over 140 slides. 

The customer has now asked for a back button, to skip back a layer, this is where I have discovered an issue. There is no previous layer option in triggers? so I have to manually set up an icon to skip back? also, the audio problem has reappeared so I assume I will need to create pause media for both directions?

Could any of you lovely experts suggest any easier methods to resolve this issue? it would be very gratefully received.