Its a low pass filter so I can only think its there to reduce any residual RF or HF signals above audio range which may damage the speaker or cause unwanted modulation effects.
The data sheet indicates it is for bass boost. Note also that the inputs are reversed from the typical amplifier circuits also shown on the data sheet.
Look at the datasheet for the LM386. At the frequencies of the AM broadcast band and the 455kHz IF frequency of an AM radio the LM386 has lots of gain. The speaker and its wires will spray the amplified signal all over the radio causing it to oscillate if the lowpass filter isn't there.