When there are no any horses and the player character is accompanied by a pet dog, the dog snorts like a horse (using the sound files horse_snort_*.wav). I think soundless is still better than horse. Of course it's welcome to be replaced with a sound of whimpering dog.
These cannot be changed and are hardcoded. So if we make it soundless then horses will also be soundless. Unfortunately a limitation of Warband.
There are actually some api to change it, but it doesn't work and i wouldn't bet on TW ever fixing it now.