How do you only play a sound if it's not already playing?

I don't want overlaps of the same sound file.

Run the "trigger once while true" event under System/Special Conditions.

That's not what he means, he means "If 'boom.wav' is playing" which I don't think you can do without trickery.

reserve a channel and use it to load the sound you don't want to overlap.

Play the sound only if the channel is not playing.