Swap out mp3 samples for wav samples in chapter 20

The sound library currently crashes when attempting to play back certain mp3
files, see issue #11:
processing/processing-sound#11
We circumvent this by using WAV instead of MP3.

Author: multimentha

chp20_sound/example_20_01_playback/data/beat.mp3

@@ -11,7 +11,7 @@ SoundFile song;
 
 void setup() {
   size(480, 270);
-  song = new SoundFile(this, "beat.mp3");
+  song = new SoundFile(this, "beat.wav");
   song.play();
 }
 

chp20_sound/example_20_01_playback/data/beat.wav

@@ -13,7 +13,7 @@ void setup() {
   size(200, 200);
 
   // Load a sound file
-  song = new SoundFile(this, "beat.mp3");
+  song = new SoundFile(this, "beat.wav");
 
   // Loop the sound forever
   // (well, at least until stop() is called)

chp20_sound/example_20_01_playback/example_20_01_playback.pde

@@ -4,7 +4,7 @@ SoundFile soundFile;
 
 void setup() {
   size(200, 200); 
-  soundFile = new SoundFile(this, "beat.mp3");
+  soundFile = new SoundFile(this, "beat.wav");
   soundFile.loop();
 }