For the last event, you have to add a condition "Else".
Construct 2 events work from the top to the bottom.
So when you click the button, and if Sound_var = 1, it changes it to 0. -> This means the next line of code is now true, because Sound_var now equals 0.
Else condition, on the other hand, runs the second part only if the first part wasn't true, and vice-versa.