Using PCM audio format you can using an audio editing program on your pc to filter and create a clean a short sample for all the leters, then using one serial/i2c/spi eeprom or flash memory or even one SDCard and just use the SDCard lins out there put each sound in a separate file and using one timer you can do a direct load of each audio sample to the timer, then just use an RC low pass filter in the timer output pin and you are done, no perfect but you can undestand very well the sound produced this way, because thats how a lot of audio dacs work.