Is the case insensitive comparison broken for mdfind?
It's working for me (Sierra). Specifically, the form you're used to works:

$ mdfind -onlyin . 'kMDItemContentTypeTree == "public.audio" &amp;&amp; kMDItemTitle == "*ciao*"'c
...[non-album tracks]/Bella ciao.mp3
...Unreleased/Bella Ciao.mp3


I had no luck with the other form (... "==[c] ... "). The docs you linked to were from 2011; I found references from 2014 that use "your" form so it looks like the online docs are old and wrong! Unfortunately this suggests there's something wrong with your system, rather that with your queries. Sorry, I'm not sure what the next step would be.