use device and item names to address the item (device with name and an item with name should exist).

				
					{
   "id":"_ID_",
   "method":"hub.scenes.expressions.set",
   "params":{
      "name": "HowLoudQuad",
      "code": "return params.how_loud * 4",
      "params": {
        "device_item_names": [
            { "name":  "how_loud", "deviceName": "siren", "itemName":  "sound_level" }
       ]
     }
   }
}