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Since I can't ask the original designer, please don't quote me on this, but I read on a similar project, "the clicking of a loudspeaker may cause significant changes in the supply voltage that under certain condition may couple to the input to the circuitry and cause oscillations. Therefore each stage of the measurement amplifier needs to be well blocked with individual blocking capacitances."So I think it is there to stabilize the input power.
Motion Feedback MP3 Player
Arduino Waveform Generator Shield
Pocket Photodiode Geiger Counter from Jameco Kit Assembly