<p>Ah I see, I was confused as to what you were doing for a second, that would be what to do if you wanted to change the volumes of the voices individually. You can use a voltage divider still to drop the signal, but if you needed to boost an audio signal higher, you can simply use a standard inverting amplifier circuit using an op-amp. The rules are simple. </p>
<p>Set the voltage at the non-inverting terminal of the op amp to 4.5 volts. Use a capacitor of almost any value in series with the signal to block any DC bias the signal may be riding on, and then pass the signal through an input resistor, R1, and then to the inverting input of the op amp. Add another resistor, R2 from this input going to the output of the op-amp. Add another series capacitor on the output, and you've boosted your signal by a gain of R2/R1.
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