this link takes me to a $44.99 bracket that itself can hold up to 44
lbs and is fully adjustable. with all that adjustability i don't see a
positioning problem. maybe you do.
vertical wooden studs are probably either 16" or 24" apart. knock and
listed horizontally across the wall to find them if you don't have a
studfinder. drill small pilot holes to confirm their locations. install
brackets with some 2-1/2" deck screws or pilot holes and lag bolts into
the wood.
unless the customer must have them in some inconvenient spot between
the studs. perhaps find some decorative 3/4" x ?" x 24" wood pieces to
span the studs. speaker vibration will try to loosen a less sturdy
mounting.