The subject of your post is misleading - it should have said "activated". Asking why features are "not deactivated" is asking why they are active in the first place which may be why people skipped your post.
That said - the reason "Split Audio" is not active is because your audio is set to Event behavior. Select a frame in the audio layer and in Properties Panel Sound section, use the behavior drop-down to change it from Event to STREAM. This will embed the audio into the timeline and thereby activating the "Split Sound" feature.