IDK, but I think Delphi hasn't released a lot of information because they don't have a lot of information.
I do think they have small, key pieces that will help nail someone but aren't particularly useful in FINDING the perp. I also think the way the girls were murdered/found has likely been shared with other LE agencies around the country to help them link Delphi to other murders. I have a hunch the break in the case will be when the perp is caught elsewhere, not from a tip from the public.
I think the reason they don't release more audio is because it's disturbing or maybe simply other segments of the audio are muffled. IDK, but I'm trusting LE has done what they can with the release of the audio.
jmo