Sound pens for kids. You run your thumb nail along the length of the mechanical pencil/pen and it plays back some resemblance of a word or recording by vibrating your thumbnail with some groves cut/molded onto the surface.
I loved “something generated from something else” type things when I was a kid.
To market it, could automate a cutter to do custom sounds.
Better yet, have this as a “customization” option for electronic devices. Is that your phone? Rub a stick on it and hear your sound byte.