How to stop isochronic tone app to stop running 15 minutes after the screen goes ones dark

The app plays different length isochronic tones. The length is from 10 minutes to 8 hours. I have carefully timed when the app stops playing the tone. It is exctly15 minutes fter the screen goes dark. I need to know how to keep the tone running until they end. My pad is a Samsung galaxy s2 8 inch tablet.

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