I still cannot find the video on YouTUbe but I did find a reference to it. If I can run it down, I will sent it along. However if you go to the Google search at the top right of the screen and enter "Center Grill" or "Center Speaker" on here you will get a listing of various discussions. The two links above are from that search. Also all Generation 2 Souls have the same speaker mounts up there and any info from them should work for you. That is 2014 to 2019 cars.
Peter, be sure to pry up at the center and then left/right points on the air vent closest to the speaker cover. The air vent will then tilt down and towards you. Once that is done, it will expose 2 bolts that you need to undo before the center speaker grill will come out.