You guys are the best! Following Juz's instructions and pictures, I've just fixed my receiver speaker (bought on eBay an "original Nokia 500 700 820 920 Lumia Asha 305 306 earpiece").

By the way, I didn't know my Jolla has several torx screw sizes, not only in length but in thread. I'm afraid I have "forced" some holes (my N900 antenna died because of one of those )