All the sounds are simple .wav files which are stored in the sounds folder. You can simply replace the .wav files for the ones you like. The meaning of the .wav files are as follows:

attention.wav - General attention sound.
check.wav - Computer gives check.
checkmate.wav - Guess what ?
draw.wav - And this one ?
welcomnl.wav - Welcome sound.
ping.wav - Used to signal errors.
quit.wav - Played when program exits.
gamestart.wav - ICC/FICS game start.  (4.3)
gamestop.wav - ICC/FICS game ended. (4.3)
shout.wav - ICC/FICS shout received. (4.3)
sshout.wav - ICC/FICS c-shout received. (4.3)
tell.wav - ICC/FICS tell or say received. (4.3)
kibitz.wav ICC/FICS kibitz received. (4.3)
challenge.wav - ICC/FICS challenge received. (4.3)

Further the sounds directory contains subdirectories for each spoken voice