Instead of the full "rock speaker", why not just a recolorable speaker grill , round or rectangular, that you can partially embed in the existing rocks, walls, scenery, even paths or terrain? You could turn any rock, wall, balcony, lamp post base, etc., into a "speaker cabinet" and the part visible to the public would be what you would see anyway, without having to create a specific speaker item that might not fit the theming of the area you wish the sound.
If you wanted something that looked like those rectangular box-like outdoor speakers you see under the eves of many malls, just put the grill on a piece of rectangular scenic or building item, and *poof*, an outdoor speaker! If you wanted music from your rock garden, just add a grill into one of the rocks!
For the more obvious speakers, like those green half-buried ones, or the funnel-like or concert cabinets you see mounted on a pole, you may still want an actual "speaker object" to place. But I bet, with this group of inventive builders, they could create some pretty "legit" speakers with just the available scenery/building elements.