I'd like to alter an audio signal by 20 hz... How could I do that?

Answer 1

Do you want to modulate the audio signal?
Do you want to shift it up or down?
Go to a library and read books on Audio Engineering.

Answer 2

Assuming this is just a simple audio signal, you can modulate it with an RF signal, say 1 MHz, then use a 1.00002 MHz RF signal to demodulate it.

Answer 3

Use frequency adder circuit, common use phase locked loop IC.

Answer 4

the information is too limited to give effort