Hi,
voice signal --> amplify if needed --> analog filtering --> ADC conversion --> digital data stream --> read from ADC --> write to sdCard.
* you need to specify analog input voltage range (for ADC and amplifier, if needed)
* you need to specify analog frequency range (for analog filter and ADC sample rate)
* after each conversion you need to read the digital data and store it to sdCard (maybe you need a FIFO buffer here)
I recommend to read data from ADC and just transmit into FIFO within the ADC_conversion_finished_ISR.
Transmit data from FIFO into sdCard I´d do in main loop.
Klaus