Apparently the counter used to track the current location in audio files (mp3s at least) is a 16bit integers. At ~ 18 hours (65536 / 60 / 60 = ~18.2), the current time seek laps back to 00:00:00. While the audio file will continue to play, if you try to rewind, power off and back on... it seeks to the location indicated by the new (wrapped) counter value 00:01:01 (back at the beginning of the file).
Most of my audio books are of the longer variety and I took the advice to join them all into a single mp3 for easier playing - not considering this limitation. Guess I'll now be splitting the longer ones