the font works normally on a NBv3 I have left. The chained sounds in your videos indicate a corrupted file system in your SD card (ie it is playing bits of sounds that are misplaced in the SD card structure). It could be the card, it could be the USB writer too.
I suggest either a low level formatting or at least a slow (non quick) formatting and re install of the package. Are you on PC or mac ? Do you have a spare SD card that works ?
Just for the sake of accuracy, it's not linked to the length of the sound file being played, we have successfully tested (long ago) the playback of super long hum or fx, like several minutes. iSaber on CF plays audio tracks that long. The actual limit of the file length is somewhere around 4GB, or 25 hrs (good news, the board could actually play the whole 7 episodes dialogs, right ? #audiobooks )