Electronic – Common GND with Buck Modules

You don't need a charge pump for this. Charge pumps are used to move energy, here you're only interested in a signal (voltage). BTW, a charge pump would leave nothing left of your signal. Adding a voltage can be as simple as a resistive adder, or, more sophisticated, you can use an opamp summing amplifier.

The advantage of the opamp circuit is that, because of the virtual ground, input signals (your audio signal and the offset voltage) don't influence each other.

Note: if you want to use the digital potmeter to create a volume control, that's not a good idea. Volume control potmeters are logarithmic, this one is linear. For a volume control the CS3310 has excellent specs, like click-free switching, though it's not cheap.

The shielding ground, by which i presume you mean the Earth and chassis as so forth, doesn't necessarily require a low impedance path to the circuit. You could short them together, but if you do make sure its done at a single point, possibly close to the power supply.

You should, however ensure that it's at a fixed voltage with respect to your system ground. A resistor to the system ground shod do the trick. It would hold the chassis at close to the system ground and would also provide a more direct path to earth from any noise or discharge the chassis may experience from the environment, than going through potentially sensitive circuitry.

I am assuming that your system ground is otherwise isolated from the Earth through a transformer.