There is not really a way to perfectly pause sounds in Scratch. This is a method of doing it (explained in the notes.)
First, we need to understand how the "set pitch effect to" block works. Not only does it change the pitch of the sound, it also speeds it up or slows it down. Setting it to a really low number, like -1/0 (-Infinity) not only makes the sound play really deep, it also slows it down, almost to a stop. Now, to pause the music, you also have to set the vloume to 0% so you don't hear the music.
