This unusual Class D amplifier is assembled on a 555 chip, which is not designed to work as a low-frequency amplifier at all.To make a Class D amplifier, you need a generator that will be modulated by sound.Next, using the LC circuit, the generator signal is filtered and the sound signal is sent to the speaker.This is exactly the principle implemented in this scheme.A rectangular signal generator with a frequency of tens of kHz is assembled on the timer 555.This signal is modulated by sound.At the output, I did not put a throttle for the LC circuit, its role is apparently performed by the speaker and it sounds loud enough.Variables resistor 100k  needs to set the optimal sound and the current consumed at the same time, in my case, the current consumption was approximately 70mA at 9 Volts.