Driving 555 control voltage with DAC
The control pin of the 555 will need to be driven with a signal from a fairly low impedance source. The data sheet of the 555 will show you the range of control voltage that will be effective. If the control range needed does not match the voltage generated out of your MIDI-2-VOLTAGE converter then you may find it necessary to adjust the DAC output range accordingly. Do check the output impedance of your DACs and make sure that they are at least 10 times lower impedance then the control pin input pin of the 555's. If this is not the case you will have to use some op-amp voltage followers to buffer between the DAC and the 555.