The sound events "hit.copper_bulb" and "fall.copper_bulb" are defined in sound_definitions.json in experimental_update_announced_live2023 (before 1.21.0)/vanilla_1.21.0 (after 1.21.0), but they are not used within sounds.json. Instead, copper bulbs make the same as the step sound when hit and fallen upon. This results in no audible difference.

Copper bulb entry in sounds.json:

    "copper_bulb": {
      "volume": 1.0,
      "pitch": 1.0,
      "events": {
        "default": "",
        "item.use.on": {
          "sound": "step.copper_bulb"
        },
        "break": {
          "sound": "break.copper_bulb"
        },
        "hit": {
          "sound": "step.copper_bulb"
        },
        "place": {
          "sound": "place.copper_bulb"
        }
      }
    },

Note:
fall.copper_bulb, hit.copper_bulb and step.copper_bulb are functionally the same, as they all use step sounds.