It's a simple program that plays two waveforms, and changes the frequency based on mouse position. You can toggle various filters with the number keys. Enjoy!

2. import ddf.minim.*;
3. import ddf.minim.signals.*;
4. import ddf.minim.analysis.*;
5. import ddf.minim.effects.*;
6. 
   
7. 
   
8. Minim minim;
9. 
   
10. //Audio signal
11. AudioOutput out;
12. 
    
13. //Waveforms
14. SquareWave square;
15. SawWave saw;
16. 
    
17. //Effects
18. LowPassSP lowSP;
19. LowPassFS lowFS;
20. BandPass band;
21. HighPassSP highSP;
22. NotchFilter notch;
23. 
    
24. 
    
25. //Font for text
26. PFont font;
27. 
    
28. void setup(){
29.   size (1000, 500);
30.   font = loadFont("CenturySchoolbook-28.vlw");
31.   textFont(font);
32.   minim = new Minim(this);
33.   out = minim.getLineOut();
34.   
35.   // create a square and saw waves with a frequency of 440 Hz, 
36.   // an amplitude of 1 and the sample rate at 44
37.   square = new SquareWave(500, 1, 44100);
38.   saw = new SawWave(210, 1, 44100);
39.   
40.   // create the filters with various settings
41.   lowFS = new LowPassFS(200, 44100);
42.   lowSP = new LowPassSP(200, 44100);
43.   highSP = new HighPassSP(800, 44100);
44.   band = new BandPass(300, 200, 44100);
45.   notch = new NotchFilter(200, 200, 44100);
46.   
47.   
48.   // attach signals to output
49.   out.addSignal(square);
50.   out.addSignal(saw);
51.   }
52.   
53.   // Prints the instructions and draws the waveform.
54.   void draw(){
55.     stroke(#FFFF00);
56.     background(100); 
57.       text("There are 2 tones being played.\n"
58.   + "The X mouse axis changes the pitch of the square wave.\n"
59.   + "The Y mouse axis changes the pitch of the saw wave.\n"
60.   + "Press 1, 2, 3, 4 or 5 to toggle low-pass SP filter, high-pass SP filter,\n"
61.   + "low-pass FS filter, band pass filter, or notch filter, respectively.\n"
62.   + "The waveform below is updated in real time. ", 15, 30);
63.     
64.    // To draw the waveform, I took the frequency values returned by the audio out
65.    // and connected them with a line. I had to multiply each by 50 to make sure it
66.    // showed sufficient differentiation between values.
67.     for(int i = 0; i < out.bufferSize() - 1; i++)
68.     {
69.     line(i, 300 + out.left.get(i)*50, i+1, 300 + out.left.get(i+1)*50);
70.     line(i, 400 + out.right.get(i)*50, i+1, 400 + out.right.get(i+1)*50);
71.     }
72.   }
73.   
74.   //Sets the frequency of the waveforms based on mouse pointer location
75.   void mouseMoved() {
76.     square.setFreq(200+mouseX);
77.     //square.setPan((float)((mouseX/400)-1));
78.     saw.setFreq(200+mouseY);
79.     //saw.setPan((float)((mouseY/250)-1));
80.   }
81.   
82. // Listens for key presses and toggles the appropriate effect  
83. void keyPressed() {
84.   if (key == '1') {
85.    toggleEffect(lowSP);
86.   }
87.   else if (key == '2'){
88.    toggleEffect(highSP);
89.  } 
90.  else if (key == '3'){
91.    toggleEffect(lowFS);
92.  } 
93.  else if (key == '4'){
94.    toggleEffect(band);
95.  } 
96.  else if (key == '5'){
97.    toggleEffect(notch);
98.  } 
99. }
100. // Turns the effect on or off
101. void toggleEffect(AudioEffect effect){
102.     if (out.hasEffect(effect)){
103.       out.removeEffect(effect);
104.     }
105.     else{
106.       out.addEffect(effect);
107.     }
108.   }
109.   
110.   // Stops the program
111.   void stop(){
112.     out.close();
113.     minim.stop();
114.     super.stop();
115. }

Written by David Shefchik