Tag the wall hit "wallhit" when playing it. Then in the collision event add another condition (audio: is tag playing "wallhit") and invert it. This way it will only play, when the wall hit sound is not already playing.
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mediafire. com /?xe4342onz9u56g4 Construct 2 admins wont allow me to post link so paste this in and close in the spaces
first event is my suggestion; second event might be what you were trying with your events, I'm not sure. Use one and see what it does (don't use both simultaneously)