The thing about your mirror is all the weight is pulling perpendicular to the wall anchor. That's where it's strongest. When using a wall anchor on a speaker mount, much of the force (at least on the top-most screw) will be acting to try to pull the anchor straight out from the wall. The Axiom FMB mounts attach to the wall at two points.
Good point, especially if using the FMB. What I envisioned were anchor points near the ends of the speaker (ie. distributed more horizontally rather than being centered vertically).
Perhaps he can attach a shelf (with L brackets) to the wall using these anchors, and then sit the speaker on the shelf. It may not be the prettiest solution though...
