All sounds are stored on the microSD card which can be easily accessed by removing left “maintenance” side door which only held by 3 simple screws. You can remove microSD card and replace existing Alarm WAV files (named alarm1.wav, alarm2.wav, etc.) with your own.
Audio files must be in following format: uncompressed WAV, 22Khz, Mono, 16 bit.
Also rather then overwriting alarm files, I highly recommend copying all WAV files from microSD card to your computer first, replacing alarm files (locally on your computer), formatting microSD card and copying old and new files back to it. This will avoid potential issue with file fragmentation.