// get the priority of current incident
var currentIncPriority =  current.getValue("priority");
// define an array to map priority with ringtone
var customisedRingtones = [
      {
            "priority": "1",
            "ringtone": "default"
        },
        {
            "priority": "2",
            "ringtone": "default"
        },
        {
            "priority": "3",
            "ringtone": "default"
        },
        {
            "priority": "4",
            "ringtone": "default"
        },
        {
            "priority": "5",
            "ringtone": "default"
        }];
//override the json["aps"]["sound"] param with the ringtone defined for current priority        
        if(customisedRingtones){
            for(var i=0;i<customisedRingtones.length;i++){
               if(customisedRingtones[i].priority && customisedRingtones[i].priority === currentIncPriority){
               json["aps"]["sound"] = customisedRingtones[i].ringtone;
                break;
               }
            }
        }