I'd like to add a potentiometer to control the overall volume output, but wouldn't know where to connect it to (respectively have tried any possible connection I could come up with).
You should connect an output coupling capacitor of about 330nF (0.33uF) to block the DC from the circuit's output from affecting the 10k volume control.
You should connect an output coupling capacitor of about 330nF (0.33uF) to block the DC from the circuit's output from affecting the 10k volume control.
The output impedance of the Cmos circuit is about 1k ohms. Adding the 10k volume control then the total resistance is 11k ohms and a 330nF capacitor will produce a roll-off at 44Hz which is low enough. A 1uf electrolytic capacitor will produce a rolloff at 14Hz which might be too low.