Region sounds don't play at the right interval
Morrowind.ini contains a Minimum and Maximum Time Between Environmental Sounds, these should be used to determine the number of seconds between sounds. Note that this is the time between the end of the current sound and the start of the next.
Given a region with a single sound effect with a chance of 1, Morrowind.exe has a 1% chance of playing this sound (and a 99% chance of playing nothing). OpenMW currently has a 100% chance of playing the sound.