Many thanks for your reply.
I forgot to mention that the plane spawns randomly so at any one time there could be two planes on the layout, and another one is definitely on its way. So, I did what you said in the two ways shown below:
In A it plays correctly, but just for the first plane. The following planes don't play the sound.
In B it plays the sound for the first plane only but continually, as in A in my first post.
Getting there, but still doing it wrong.
Edit: I also tried this, but that also didn't do the trick.
It was the same result as A in this post: