[gregwtmtno]

You used to be able to do this by showing certain images on a CRT. It was cool trick back in the day.

[JNRowe]

Tempest for Eliza¹ was doing the rounds some years ago. I'm sure there were other implementations in the wild, but I can't remember their names.

1. http://www.erikyyy.de/tempest/

[gregwtmtno]

Yup! That's the one I remembered. On a side note, I wish they still made sites that look like that one.

[ido]

this was also the way to generate audio in old trs80 games (the Model-I had terrible RF shielding).