Published var in Class not available in array as a method

I am creating an array of SpriteNodes which each have an instance of another binded variable which is actually an instance of synthesiser code built with AudioKit as so...

var ballArray: [SKShapeNode] = []
var numberOfBalls = -1

class DroneBall: SKShapeNode, ObservableObject {
  
    @Published var conductor = DynamicOscillatorConductor()
    
    override init() {
          super.init()
      }

      convenience init(width: CGFloat, point: CGPoint) {
          self.init()
          self.init(circleOfRadius: width/2)
          fillColor = .red
          strokeColor = .yellow
       }

      required init?(coder aDecoder: NSCoder) {
          fatalError("init(coder:) has not been implemented")
      }
}

I create each instance of this class in a function within a SpriteKit scene and add the ShapeNode to the scene.

 func buildABall(location: CGPoint) {
        let ball = DroneBall(width: 100, point: location)
        ball.conductor.data.frequency = AUValue((location.x * 10))
        ball.conductor.data.isPlaying = true
        ball.conductor.start()
        ballArray.append(ball)
        numberOfBalls += 1
        ballArray[numberOfBalls].position = location
        addChild(ballArray[numberOfBalls])
        
        
    }

The code is correctly creating the SpriteNode and will add to the scene each Sprite each one with the Synth running. However I am unable to access methods of the synth on the array such as

ballArray[numberOfBalls].conductor.data.frequency = 400

I get the error: Value of type 'SKShapeNode' has no member 'conductor'

So if it is there in the SpriteNode before being added to the array why is it not also in the array? I need to be able to access the values of 'conductor' elsewhere in the code so I can change the variables within.

Does the array also have to be an observable object in some way?

Declare array of objects which you place in it, like

var ballArray: [DroneBall] = []