I tried editing sound.json and blockmap.json to revert some blocks to their vanilla sounds, but it didn't work—the sounds just stopped playing. Please tell me how to revert only specific blocks to their vanilla sounds.

The wiki states that if a block is unassigned (literally "no mapping exists"), PF will continue searching for a mapping. If none is found, it says the default Minecraft sound will play. So I tried defining a block in Blockmap.json as UNASSIGNED, but only the footsteps disappeared—the vanilla sound didn't play. What does it mean for a mapping to be "not found"? I'd appreciate your insights.
I apologize for my poor English as I am Japanese.