Powering 12V 2A device from high-power USB portable charger
5V * 4.8A = 24 Watts.

12V * 2A = 24 Watts

Since no step-up regulator or boost converter can do 100% efficient, you can't do this. Even assuming 90% instead of the typical 80%, that's 24 W * 1.1 = 26.4 W / 5 V = 5.3 Amps in. Half an Amp more than you can provide.

Now if your 12V device can run at a lower voltage or a lower current or both, it's possible. Use an ammeter with the amplifier at full strength to see what the peak current drawn actually is.

Consider getting a smaller or more power efficient amplifier, or a power bank that's designed for 12V and 5V out.