it's deffinatly a .nif problem. I've had it once myself. I think i had linked the unit details to a dds, or something stupid like that.... or to a slightly wrongly named file, or path....
The one thing i DO remember, is that it was a very easy fix. Just double check all the units that are effected in your artdefines sheet, then double check your .nif's, in the units folder.
The fact that sometimes your units DO appear, confuses me...... but i still blame your .nif files. If i were you, i would probably re create, all effected units... but that's me..
as for your sound problem. no idea.