Problem with a Butterworth filter in stereo to mono audio input
R1 and R2 load each other's output stages.  For example, if the instantaneous voltage to the left of R1 is +1.0 V and the instantaneous voltage to the left of R2 is -1.0 V, the voltage to the C-R filter is 0 V.  If the left channel is at full volume and the right channel is at zero volume, the voltage to the filter is -6 dB relative to the inputs because the two resistors form a voltage divider.

Also, since there is only one reactive element (C1), the filter performance is 6 dB / octave, not 12 dB / octave.