Changing the volume on SCNAudioPlayer

So, I have this little game I'm trying to build and inside it there are some pieces of wood that can slide on top of a bigger one.

I want to lower the "slide audio" volume based on the velocity of the wooden piece, but I can't seem to be able to change the output volume no matter what.

To play my sound I use an SCNAudioPlayer and attach it to the node like this:

let player = SCNAudioPlayer(source: source)           
player.willStartPlayback = {
     self.audioPlayersTracked[node] = player
}
player.didFinishPlayback = {
     self.audioPlayersTracked[node] = nil
}
node.addAudioPlayer(player)

Now everytime the physics is simulated I loop trough the "tracked" audio players and change it's volume based on the velocity of the sliding piece:

func renderer(renderer: SCNSceneRenderer, didSimulatePhysicsAtTime time: NSTimeInterval) {
     for (node, player) in audioPlayersTracked {
          var speed = velocityForNode(node)
          if speed < 0.01 {
               speed = 0.0
          }
          
          let volume = max(0.0, min(1.0, speed))
          player.audioNode?.engine?.mainMixerNode.volume = volume
          print("Did set volume \(volume) @ speed \(speed)")
     }
}

My logs show up alright. But the volume never changes. Any idea how is this supposed to be done?

P.S: The SCNAudioSource i init my player with has a volume property. But trying to change that doesn't work either. In fact for all properties of the SCNAudioSource the documentation mentions that if you need to change it "real-time" you should access the audioNode property of the SCNAudioPlayer