Damian wrote in wondering how the MidiVox can be used as a MIDI to control voltage converter. I figure my response might shed some light on the topic for others as well -

I hope to have an example MIDI->CV sketch up soon, but suffice it to say -
a DAC (such as the MCP4921 chip) outputs a specific voltage level set by according to data sent out by the Arduino (or other microcontroller).  The example synth sketch I have up now continuously sets the DAC's output to varying levels to create a waveform, (the speed or frequency at which it does this determines the pitch of the note).  To use the DAC as a control voltage generator we only need to set the DAC to one specific voltage level for every MIDI note - no need to be concerned with any specific frequencies.

Example: 
if (incomingNote = C4) outgoingVoltage = 3.250;
if (incomingNote = D4) outgoingVoltage = 3.417;

… and when no note is being played, we turn the outgoing voltage to zero.

Anyway, that's the basic idea.  In order to be used with analog synthesizers and the like, we’ll probably need to solder a wire/connection to one of the digital output pads on the shield and set that pin high whenever a note is being played (aka - Gate)There’s also a jumper on the board labelled "AUD/CV", which disables the 10uF DC-blocking capacitor - which needs to be set to (yup, you guessed it) “CV” to enable control voltage out ;)