So I want to take issue with one thing you said, that REM is “…not measuring the sounds that actually help her to understand speech.” The REM machine that my audiologist uses is speech based and it has prerecorded speech at multiple volumes that hit a specific mix of speech sounds (phonemes) to ensure that the hearing aids are producing speech sounds in line with one’s prescription. Now, no, that doesn’t tell you what her brain is processing, but it is a start, and it’s a lot more than just BEEP.
Plus, I’m now an expert on carrots (the REM sentences are all about carrots).