What is the purpose of this AND gate in an audio amplifier
We can only guess, but it is most likely a buffer, both to speed it up and to reference it to the 3.3V supply going into L1.

If I were implementing something like that, I'd make sure that the 3.3V supply was exceptionally clean, possibly even regulated from some higher voltage for no other purpose than to provide power for that AND gate.  The reason for this is that the AND gate is essentially multiplying its supply voltage by the PWM -- that's why I'd make it clean, and that's why the circuit designer has put L1 and C27 in there.