cleanup examples + removing unnecessary callbacks

I had a couple private callbacks that made it possible for users to do things like this:

```
function setup(){
  sound = loadSound('sound.mp3');
  sound.play();
}
```
If play() was called before a sound loads, the play() method would load the sound buffer with the private play_now() function as the callback. I thought this could be a useful alternative to preload(), and it was a fun experiment. But I think this hides what's going on behind the scenes, rather than teach users about the way JavaScript works. Preload() is much more effective, and another option could just be confusing, so I'm removing this little experiment.