I wrote a library to record speech quality audio using the Arduino internal ADC http://code.google.com/p/waverp/
WaveRP is an Arduino library for recording and playing Wave files with the Adafruit Wave Shield. It records 8-bit mono files at 4,000 to 44,100 samples per second.
Use of the Wave record/play library, WaveRP, requires the following:
Arduino with a 5 volt 328 processor.
Low noise power source such as a nine volt DC adapter or battery.
Adafruit Wave Shield (version 1.1 is best but 1.0 works)
Microphone preamp. A circuit for a simple preamp is included in the documentation.
Microphone, PC type with 3.5 mm plug. See the documentation for details.
SD/SDHC formatted with 32KB allocation units.
You don't need the Wave Shield if you only want to record. The Wave Shield has a DAC and amplifier for playing Wave files in addition to an SD interface.