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If the sound is still going from the first one - the next one will overlap. I'm trying to make the sound stop when I click a forward or back arrow while the next sound plays.
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please only post the code relevant to your problem. Your code consists largely of volume control features which do not seem to be part of your problem. When you post code that is not relevant to the problem, people will wonder if you understand the code you are using, and consequently not understand their suggested code solution, and may not put forth the effort to help.

Also it is frustrating for someone to read about a problem, such as forward back buttons, and not see any code related to forward or back.

So, I would suggest that in the future, if you are hoping for people to help you, please consider that you need to provide relevant information in your post.