I recall that I played with this some years ago.
Afaik there were problems with the filesize that was too big.
I worked arround this by downloading the files from the device (in fact extracting it from a fw-upgrade image), opening it in audacity and then replacing the audio by something I wanted. This was then saved in the format the original file had.
It's very important that the files remain as tiny as the oringal ones are (only some kb big). So you have to keep quality and length as low as possible.