Re: how to overcome noise caused by dot matrix scanning effe
The most likely place for the noise to be coupled is through the power supply. As the LED's are switched on and off, you are creating power supply spikes that could easily be coupled through the common power supply connection to the audio stage. A preamplifier is a very likely place for such noise to be picked up because of the high audio gain normally associated with such a stage (but it is not the only possible place).
Try filtering the power supply. You should take a look at the power supply with an oscilloscope to see the ripple while scanning. Once you have an idea of the frequency range of the ripple, you can design your filter for optimum results.
If you don't have a scope available, to make a quick check you could tack a 10uF (or some other large value) capacitor in parallel with a .1uF (or something between .01 and 1.0) capacitor and place it across the power supply at the audio stage. If the intensity and/or frequency of the noise changes, you have confirmed the source.