How to put a music track behind more than one screen

I've just figured out a way to put a music track behind a small scene with 5 screens in it, so that the music plays for the whole timeline.

Keep screen 1 as the base layer and make the next 4 screens into layers 1 to 4. Count the total timeline for the base and all of the layers and stretch the timeline on the base to that total time. Insert the music track on the base layer. Use audacity or camtasia (or another editor) to make the music track the same duration as the timeline on the base layer. It works well to fade the music at the end.

Put in triggers on the base slide saying when to show each layer, according to the duration of the timeline of that layer. Don't check the option to hide objects on the base layer, otherwise you'll block out the audio file. Does anyone know of another way? 

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