Audio Mod explained

As the manual states, the "Audio Mod" parameter in the filter section controls the amount of audio from oscillator 1 to modulate the filter's cutoff frequency. 

If oscillator 1 is playing a C1 note, it's frequency will be ~32 cycles per second (hz). If the "Audio Mod" parameter is turned up, the filter's cutoff frequency will be modulated at oscillator 1's rate of ~32 cycles per second. If you change the tuning of oscillator 1 to a higher note value, the filter will now be modulated at a faster rate. For example playing C1 will modulate the filter ~261 times a second. 

Using audio rate modulation sources (source rate > 20hz) allows you to create many different timbres of sounds that are not possible solely using subtractive synthesis. Using the "Audio Mod" parameter will allow you to make metallic and bell like FM sounds. You can read more about audio rate modulation and FM synthesis at the following links:

http://en.wikipedia.org/wiki/Frequency_modulation_synthesis
http://www.indiana.edu/~emusic/fm/fm.htm