For instance, in "アカウントにサインイン中です。" How is "中" pronounced?
At a purely mental level, I've always maintained the idea that how you read is up to you, which seems to be backed by stylistic differences in furigana usage. (Just... as an example.) However, there does come a time, even in the hermit and/or quiet person's life when one will need to speak and be understood!
Sorry for the monologue. Anyway, my educated guess is that it is pronounced なか, although my brain seems to lean more towards じゅう when not reading aloud. Bonus points (Not sure how they would be awarded.) if you can provide counter-examples-- given they exist!