SMPS --> TDA8920 | +V GND -V op-amp
How do I wire it all up?


Towards the back of the data sheet it gives you a typical circuit to use and this shows where GND comes in: -



Note the incoming power arrives on the circuit at the top left (red rectangle).

The ampere rating of your power supply depends on how the speakers are configured and what supply voltage you decide to go for. This last part dictates how much power you deliver to the speakers. Once you have the power, you can assume the chip requires 20% more then calculate current from this i.e. Iin = Pin / V.