Maybe it was designed by people who couldn't hear it? High-pitched sounds are supposedly the first to go with age/tinnitus/etc: https://mosquitoloiteringsolutions.com/why-mosquito/sound-de...
This technology required a high-voltage (~100V) squarewave across the mostly-capacitive EL panel. This squarewave was at audio frequencies.
The panel also happened to function as a piezo tweeter, depending on mounting.
I worked for a cell phone company at the time. I have two memories of this technology:
1. Using the prototype phone out of its case and touching the 100 V squarewave to my ear. That's some tender skin...
2. Using a primary transformer in reverse as an audio transformer, to play the lab stereo over EL panel. It turns out the panel color changes a little bit with frequency.