When you switch to voice mode and issue the voice receive command, the modem's internal ADC samples the audio data on the selected input (phone line or microphone input jack) and sends on the serial line the raw digital audio data encoded in G.721, GSM or IMA ADPCM format. It is designed to work with a serial port. It doesn't involve any analogic signal between the modem and the device connected to the serial line. The modem handles the analogic part.
I don't mean to be rude but I think you haven't read a single word of the file I sent you because if you go to the voice mode part it is quite clear that the audio is sampled and sent encoded in a digital format to the computer connected to the serial line.
I'm not sure about the SD card part... The speed needed is not much (about 38400 baud with an 8kHz sample rate on the modem) but I don't remember what the SD writing speeds are.