Thanks for your answers!
I'm thinking my TV set (Samsung QE65Q60R) could be faulty because if I try my audio extractor with a simple PC monitor, it works, I get an image on the display. I also tried it on my TV with my BluRay player: my "smart" TV complains about an unknown source (on same HDMI port), but fiddling with the remote I can force it and finally get an image on TV. Yet, no spdif sound is extracted...
My extractor provides output drivers to b00st (censored??) output signals and allow longer hdmi cables. So it's not passive and it might be defective. I've obtained an RMA. I'll return it and try again with the replacement one...
On TV side, I tried the spdif output with passthrough function like on the Sony TV. I got audio on the AVR, but only 2-channels (i.e. Pro Logic and the like). I also tried the HDMI ARC output connected to my extractor just to get the spdif audio, without connecting the HDMI output. As expected it failed.
I'm please to hear that USB->SPDIF devices work with Kodi. Given the low price I think I'll order one like Iyoko as a plan B. Anyway, SPDIF as well as my Yamaha amp don't support HD audio so it's worth a try...
But, I also have in mind the fact that the obsolete element in my home system is the AVR. I'm currently struggling to have all my recent devices work with my old Yamaha. Maybe is it time to stop wasting money on thinkered solutions and shell out the money for a fully HDMI, beautiful new AVR... 