This isn't a dev question, but ...
The simplest solution is to get something like Audacity or an online visualizer (
Microphone Sound Analyzer[
^] is one) and genuinely display the mic input ... if the mic has a mute button she can operate that would prevent "background" distorting the voice display when she isn't speaking.
Good luck with the "40 feet away" bit - you're going to need a pretty big display!