How do you remove the speaker grills on a 928?

Depends upon the year and the audio system installed... Some of the round grilles pop off, and some screw off. It is really difficult to visually tell the difference, and you can break the grille if you do the wrong thing. I would suggest first trying to unscrew the grilles - standard lefty-loosey. It is possible, but not too likely that the grille will break if it is a pop-off version, while if you pry on a screw-off version, you WILL break it. If you do break the grille rim, you can repair it with electrical tape. Get some GOOD electrical tape, such as 3M 33 in the standard 3/4" width. Fit the edges of the ring together, then apply tape without stretching it, starting about four inches from the break. As you get to the break, start stretching the tape, keeping one edge on the edge of the flat part of the rim. Make a full turn while stretching the tape, then continue applying the tape while aligning the edges, but stop stretching the tape. Apply about six more inches without stretching it, then neatly cut it off. Roll the other edge of the tape down over the rolled rim as neatly as you can, then trim the excess tape. Properly done, the repair is almost invisible, and will last for a couple of years or more.