Role of input resistance in LM384 Audio Power Amplifier
It provides a path for the DC bias current to the input transistor so that the signal can be AC coupled with a capacitor.

Note that in the application diagram (fig 13 onwards in the data sheet) the two inputs can be connected to ground - it is not required to provide a bias voltage.

The value of the resistance, 150k, is high enough that it does not have any significant affect on the circuit operation.