The branch note is an equalent to an IF so you want to check something. You check if your variable is true or false.
When do you want your sound to play or when do you don’t want it to play?
-You want it to only play once. So you want to set your “stop” to true when the sound was played the first time…
(I asume that you are not that much familiar with programming, so I will only point you to the right direction, but you need to do the last step 
)