The triggered once is unnecessary, because the 'one pressed' event is a triggered event (green arrow).
It will trigger every time you press z but only once, even if the sound is playing. If you want that it only plays when the sound has ended you have to use a second condition.
Give your sound a tag-name, something like FlipperSound.
Keyboard| On Z Pressed -> Audio| Play flipper not looping ... (Tag"FlipperSound")
Audio|(invert) Tag "FlipperSound" is playing