Placing a full slab (half slab with another half slab on top of it) plays the player walking sound of the same block type (stone/wood).

How to produce:

Start/join a world.

Equip any form of a slab block (for example oak slab)

Place down the slab.

Place down another one on top of it

You'll notice that on when you place the half slab, it'll play the block placing sound (normally) but when you place another slab on top of it to make a full slab then you will hear the same sound you would hear if you walk on that block.