Sort of the blind leading the blind here (I'm not actively creating GECK mods, but we don't have a regular animation contributor), but as I interpret the wiki:
You need to open the "Idle Animations Window", locate and open the "Animation Group Selection" portion of the data (right-hand) part of the window, and select the "Special Idle" group there. Add your new idle KF file to the list?
According to the "Sounds Tab" page:
Inherits Sounds From: if this field is filled out, the table cannot be edited and all of that creature's sound data will come from its currently selected inherited list. In order to create a creature with completely unique sounds (or to creature a template from which other creature's will inherit all of their sounds), leave this dropdown list set to 'NONE'.
So it sort of depends upon the "creature" you are talking about. Did you "clone" one by changing the "Editor ID" of an existing creature (which would use inheritance of the "Base ID" form of the original creature), or create a new one from scratch (meaning create a new Base-ID form) and give that new form a "Sound Data Table"? (Please see the 'GECK Form-ID, Base-ID, Ref-ID, and Editor-ID' section of the wiki "Getting started creating mods using GECK" article if you are not clear on those terms.)
I would think the sound file would be linked in the KF file so it was tied to the animation duration. Isn't that covered in one of the tutorials in the "Animation" section of that same article?
Edited by dubiousintent, 20 July 2020 - 02:56 PM.