If sound walls, those aren't going anywhere, even with some load. I'm assuming there'll probably be a screw or two securing the supports to the shelves too.
Bottom shelf 2 boards I cut a track around the outside of 20x20mm I then used 20mm aluminium box which measures 18x18 I fixed this which is a u shape and is fixed to the wall on 3 sides I've yet to fix the boards to the box as I may oil the wood you could stand on the shelf if needed, the box can't be seen unless you're lying on the floor,
The top shelf I used the offcut of wood from the board to support it