This is a simple oscillator with multiple resistors in serie. When you press any switch, the circuit starts oscillating. You can user variable resistors instead the 1k resistors. Using variable resistors you will be able to tune the frecuency for each note. You cannot be able to press two keys at the same time because the frecuency will change. To make an electronic organ capable to press many keys at the same time, you may need to use thirteen 555 ICs. Is ok to use 9 to 12V as power but it may be quite loud. I do not recommend to use a power supply as it may cause electrical shock. Also a `hum` will be audible while it is in use.