Calculate output of Transistor Common Emitter Amplifier
What you have is a succession of low pass RC filters operating on a square wave followed by an active filter.  As each filter takes effect, the waveform becomes rounder since its high frequency components are being reduced in magnitude.  Calculating the value of the sine wave voltage, which includes the effects of all of the filters, is a difficult paper calculation because the filters interact. Since R4, R5, and R6 are the same value, each RC stage affects the one preceding it. Your best bet is to use a circuit analysis program.