I personally don't like a whole wall plate just for one speaker wire. If you want to bring it out the wall (hard to tell from the picture how far down from the ceiling you are), just use an Arlington bushing to finish the hole nicely and poke the bar wire through to make it as unobtrusive as possible. Some speaker brackets have openings to allow the cable to pass through the interior, hiding the wiring completely.
If you want to bring the wire down the wall but will have difficulty getting to the top plate, you could pull the crown molding off to cheat. Bring the wire through the ceiling behind the molding (cutting some drywall off in the corner if you need to), then poke into the wall, again behind the molding. Then back out right behind the speaker. Not trivial, but may be easier than drilling down. On that front - look at a right-angle drill adapter to help with drilling the top plate.
The other option is to secure the wire to the wall all the way down to the bracket, and paint the wire to match both the molding and the wall color as it travels.