<p>It has to do with the fact that the voltage supply for the Drone Lab is signal ended, i.e. the available voltages are 0V (GND) and 9V (power). All sound waves are AC signals, as they move above and below a mid-level voltage called a DC bias. The frequency of this modulation is the pitch, and the amplitude is the volume. To allow for the most headroom (amplitude variation without hitting the max or min voltage supply and creating a flat edge in the sound wave), the bias point is usually selected to be half way between the min and max voltages available. To create this bias point, simply connect two high value resistors (large so they don't waste energy due to ohmic heating, 100k or so should be fine) in series with the 9V supply to ground, and the voltage at the point where the two resistors connect should be half the voltage, in this case 4.5 VDC.  Ideally, the op-amp should not pull current frm this voltage source through the non-inverting input terminal, but if you run into any problems, try using a small decoupling capacitor, 0.1uF should be fine, by tying one end of it to the 4.5V voltage source and the other end to ground. This adds a bit of "voltage momentum" to prevent oscillations. </p>
<p>All of the explanations I am giving on this thread are sort of "quick and dirty", and I'd highly suggest picking up a book on beginning electronic audio design for more depth and clarity. I personally suggest The Art of Electronics. But with these basics I think you can build the circuit you want, and begin to get a good feeling for intuitive amplifier design with a little trial and error.
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