LoadSound("it file").play(true) didn't work, but I think I remember this being an Allegro 5.0 specific problem, not a 5.1 one.
this.size = Math.round(4+Math.random()*7);
RNG.range(min, max); Returns a random integer uniformly distributed within the range [min,max].
this.size = RNG.range(4, 11);
Actually? Don't fix that. In hindsight, what you said about throwing the reference away, it might actually be good for the engine to fail at accomplishing this so people will be forced to do it the 'right' way instead.