I know it's a lot of extra work and it takes some time, but I would remove a section of the sheetrock and install a stud where I needed it, then repair the sheetrock. I would use some L brackets on the 2x4 made for this purpose and screws. Since you probably can't add a stud that goes all the way from top to bottom of the wall, I would first add a 2x4 as a header board that spans 2 studs, above the spot where you are going to mount the speaker, then a shorter stud that goes from this board to the bottom, all attached with L brackets and screws. Then once that's in place and solid, add the piece of sheetrock that you removed back and screwed in place. When removing the sheetrock, make sure that you remove a piece that's wide enough that you have plenty of room to screw it back into the studs. (When cutting it, go an inch or 2 beyond the studs.) Then widen the cracks with a knife making a V and fill in with drywall mud, let dry, sand and paint. (Probably take about a week).
