Processing comes with a bunch of example sketches. Had you looked, you could have found this one that does Audio input. I've modified it a bit so you can see how to use the values as a point of data for doing visual effects.

1. /**
2.  * Get Line In
3.  * by Damien Di Fede.
4.  * Slight color modification by TfGuy44
5.  *  
6.  * This sketch demonstrates how to use the <code>getLineIn</code> method of 
7.  * <code>Minim</code>. This method returns an <code>AudioInput</code> object. 
8.  * An <code>AudioInput</code> represents a connection to the computer's current 
9.  * record source (usually the line-in) and is used to monitor audio coming 
10.  * from an external source. There are five versions of <code>getLineIn</code>:
11.  * <pre>
12.  * getLineIn()
13.  * getLineIn(int type) 
14.  * getLineIn(int type, int bufferSize) 
15.  * getLineIn(int type, int bufferSize, float sampleRate) 
16.  * getLineIn(int type, int bufferSize, float sampleRate, int bitDepth)  
17.  * </pre>
18.  * The value you can use for <code>type</code> is either <code>Minim.MONO</code> 
19.  * or <code>Minim.STEREO</code>. <code>bufferSize</code> specifies how large 
20.  * you want the sample buffer to be, <code>sampleRate</code> specifies the 
21.  * sample rate you want to monitor at, and <code>bitDepth</code> specifies what 
22.  * bit depth you want to monitor at. <code>type</code> defaults to <code>Minim.STEREO</code>,
23.  * <code>bufferSize</code> defaults to 1024, <code>sampleRate</code> defaults to 
24.  * 44100, and <code>bitDepth</code> defaults to 16. If an <code>AudioInput</code> 
25.  * cannot be created with the properties you request, <code>Minim</code> will report 
26.  * an error and return <code>null</code>.
27.  * 
28.  * When you run your sketch as an applet you will need to sign it in order to get an input. 
29.  * 
30.  * Before you exit your sketch make sure you call the <code>close</code> method 
31.  * of any <code>AudioInput</code>'s you have received from <code>getLineIn</code>.
32.  */
33. 
    
34. import ddf.minim.*;
35. 
    
36. Minim minim;
37. AudioInput in;
38. color white;
39. 
    
40. void setup()
41. {
42.   size(512, 200, P2D);
43.   white = color(255);
44.   colorMode(HSB,100);
45.   minim = new Minim(this);
46.   minim.debugOn();
47.   
48.   // get a line in from Minim, default bit depth is 16
49.   in = minim.getLineIn(Minim.STEREO, 512);
50.   background(0);
51. }
52. 
    
53. void draw()
54. {
55.   background(0);
56.   // draw the waveforms
57.   for(int i = 0; i < in.bufferSize() - 1; i++)
58.   {
59.     stroke((1+in.left.get(i))*50,100,100);
60.     line(i, 50 + in.left.get(i)*50, i+1, 50 + in.left.get(i+1)*50);
61.     stroke(white);
62.     line(i, 150 + in.right.get(i)*50, i+1, 150 + in.right.get(i+1)*50);
63.   }
64. }
65. 
    
66. 
    
67. void stop()
68. {
69.   // always close Minim audio classes when you are done with them
70.   in.close();
71.   minim.stop();
72.   super.stop();
73. }

This is what I was looking for!