MODEM-IN
  1=GND 2=MIC1 3=GND 4=MONO_IN

AUX1-IN
  1=AUX1_R 2=GND 3=GND 4=AUX1_L

CD-IN
  1=CD_R 2=GND 3=GND 4=CD_L

The MIC1 line on MODEM-IN is connected to the MIC1 line at MIC-IN.
So you could actually have simultaneous inputs to the CODEC. 

The MONO_IN line is split into AUX(R/L)2 inputs of the CODEC, so the
MONO_in is actually another AUX input.