Hi Jason,
On TLV320AIC3204, the only way to do the mono mix is in analog PGA. You can configure Page 1 / Register 52 to mix, say IN1_L and IN1_R using 20k resistors into MicPGA_LP and Page 1 / Register 54 to connect CM1L using a 10k resistor into MicPGA_LM. This will effectively apply -6dB per channel to avoid clipping of the mono-mix. This will send the data to the left channel I2S stream only.
If you have questions regarding the PurePath Wireless device, please post to the appropriate forum.
Thanks,
J-