Or you could make a global variable that tracks if the button was clicked, where on its first click, it'll set it to 1 and allow a song to play, but if you click again, it is set to 2 and won't play.
Try "On button clicked -> Sys: Add 1 to variable -> | if variable = 1 -> play sound"
That would make it nescessary to add a second event to set the variable back on sound end, because he wants the sound to be playable again when ended..
Just using one condition and not adding a variable seems a lot easier..