When a person reads between the lines, they are inferring meaning which is not explicitly represented. What is an idiomatic version of this which can apply to spoken words?
Vis–à–vis something similar to describing a speaker's attitude as an “undercurrent” of whatever.
BTW, something like ‘to hear that which was not said’ or the like would probably not be a proper idiom.
P.S. I've seen the question 'Between the lines' or 'read between the lines', and this is not a duplicate of that by any interpretation.