That was the era of the elecroluminescent backlight.

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.