The Audio Mixer does just that: it brings together the signals from the three VCOs at levels you select. Of course, because this is a synthesizer and not a traditional mixing console, you can modulate these levels using MIDI control to produce an ever-changing mix.
The Audio Mixer brings together four input signals:
Either the Noise Generator or the Ring Modulator
VCO 1
VCO 2
VCO 3
For each of the VCOs, you can select either the sawtooth or pulse wave.
The Ring Modulator is an audio source created by (funnily enough!) ring modulation of VCO 1 and VCO 2. Ring modulation is a form of amplitude modulation, where if a signal of frequency f1 is combined with a frequency f2, the ring modulator outputs two new frequencies: (f1 + f2) and (f1 – f2).
Since these frequencies are being linearly added and subtracted (rather than exponentially, as with pitch), the resulting frequencies usually have no harmonic relationship to the inputs! This produces metallic, ringing inharmonic tones that can add a powerful extra something to a sound... or reduce it to a clanging pile of what guitarists (who inexplicably love ring modulation stompboxes) call "gonk".
The mixed audio signal is routed to the Voltage Controlled Filter (VCF).