I forget what it's called, but I have one I got off F-Droid that just does a spectrogram of the microphone input. It's cool seeing the world yet another way.
I've taken the extraordinary step of walking a few feet to my phone, and it looks like it's just called "Audio Spectrum Analyzer".
One thing you pick up on quickly is that speech is mostly undertones, and the actual information carrying part is subtle variation in the higher parts of the sound. Singing is noticeably different, and manner of speech is also very visible.