Assuming you don't want to make invisible the objects as they're touched, your best way to keep track of the current objective would be with instance variables.
Your link gives me a 404, so I'll approximate what I mean:
Create an instance variable for each sprite, a boolean called "target" or something. Set to false to begin with. Then, along with each touch condition, create another condition that the sprite's target must be true to perform the actions.
Now what you want to do is create the mechanism for altering that boolean as you go along. For each new objective, you should nominate the next target by changing its variable to true. Then, when that object is touched, set its variable to false, play the sound and set the next objective's variable to true.
I've included a very rough capx so you can see what I mean: capx