Default creature soundgen entries are not used
Some creatures such as Ash Ghouls do not have specific footstep sounds so they don't play any sounds while walking whatsoever. In Morrowind, they do have footstep sounds. There are DEFAULT000X entries in Morrowind.esm which seem to be used for any creatures that don't have a soundgen assigned, however, OpenMW strictly uses only the sound IDs which correspond to exactly the same creature ID as the one used by the creature.
There is currently a hardcoded fallback landing sound in getSoundIdFromSndGen methods for the cases the creature doesn't have one, but it's not even the correct ID.