Skathari do not have a movement sound when wandering. about polaris HOT 1 OPEN

The turning sound instead of 'movement_sound' was intentional because movement sounds are way too constant instead of the intended effect of "distant ominous chitter echos".

But yes, this is 100% a bug. Apparently the 'turn sound' only fires when the aggro AI package is active and not just, general wander-back-and-forth AI.

I don't really recommend shoving in an entire constant sound, given spider sounds already overlap themselves a hundred times, nor is the general 'constant ambient sound' that I believe mobs also support really suitable. Using turn_sound was meant to be a workaround to get occasional sounds without adding an entire special "check when last made sound" proc or some shit, but isn't working as intended.

from polaris.