I remember when most speakers had metal grill covers. I’ve never dented my own speaker cones but here’s a CNET video showing a few tips on how to fix a dent. I would have used Scotch tape but I’m guessing it doesn’t work?
“Did someone poke in the center of your speaker? CNET’s Donald Bell shows you how to take the dents out of a damaged speaker dust cap.” – CNETTV
For more info: wikipedia.org/wiki/Loudspeaker