Strange state in input at the amplifier
The two (sort-of) triangular waveforms are AC-coupled to eliminate the ~Vdd/2 bias and prevent excessive loading of the signal. The 100K resistor and 500K pot adjust the attenuation, then the signals are combined via the diodes.

The 100K is probably to keep the oscillators from stopping entirely or behaving in a squirrely manner when the input impedance of the amplifier is low.