Function of Specific Capacitor in Audio Amplifier's Negative Feedback
The gain of the amplifier depends on Rf1 and Rf2.

As you know, the input DC offset of an amplifier, which depends on imbalance in the input transistor pair, appears at the output amplified by its gain.

C2 is a capacitor, so it does not pass DC. This removes Rf1 from the equation, and brings the gain back to 1 at DC.

This is a simple trick to make sure the output DC offset voltage is not multiplied by the amplifier's gain, that's all.