Create a voice recorder using a PIC or a microcontroller and ADC. Use some conditioning circuits to condition the microphone output and feed it to the ADC. The converted values can be stored in some kind of memory. Samsung has some nice memory chips that can be easily interfaced. Most modern MP3 players use this kind of memory. Use a DAC to convert the stored voice data back into analog format and play it through a speaker. If you feel this is very simple for you then interface a keyboard and LCD display and make it look like a real voice recorder. Give options to change the storage format, like frequency, bit rate etc. Still you are not satisfied then use an ARM processor to implement this. Convert the voice to MP3 in real time and store the MP3 data. Also write code to play back the stored MP3.