What sensor do I equal to myPort.read?
Not at all sure what that means. That code was all you need in your draw function.
Is there also anyway where I can make it so that the files can never play together
Yes you can stop one before you start another.
The if statement in that example just does the one instruction as a conditional instruction after the the if condition.
You can do as many statements as you like if you enclose the statements in braces. You have to know the code to stop a file which I don't know off hand and you have to have a variable that holds the playing sample's name so you can stop it. Something along these lines:-
if(sensor=='1') {
// stop the sample in the variable songPlaying maybe songPlaying(stop) or something like that
song1.play();
songPlaying = song1
}