 AC coupling, also known as capacitive coupling, removes the DC component from an electrical signal, so it's useful for isolating AC signals, like for an audio circuit's input. Hardware-wise this couldn't be simpler. You can add AC coupling to your circuit using just a single capacitor in series with the signal source. The capacitor works in conjunction with the circuit's total resistance to ground. Together, forming a high-pass filter and filtering out slow voltage fluctuations, the DC component, and allowing only faster AC changes to pass. Increasing the capacitor value will lower that cutoff point, letting even lower frequency changes through. For audio inputs, try starting with a 1-microfarad capacitor, and raise and lower it as needed.