AVSesssion shared and using a singleton?

Documentation states that using AVFoundation that the app get a system defined shared instance.

If i use something like this

  class var Shared:Audio
  {
    struct Singleton { static let instance:Audio = Audio() }
    return Singleton.instance
  }

or using the standard default definition

var Audio : AVAudioSession = AVAudioSession.sharedInstance()

Is there a difference in overhead? latency?

If I want to have background music playing and then an affect, would I need have two players under one session? musicPlayer and sfxPlayer?

or if I set

AVAudioSessionCategoryAmbient

is the session manager smart enough to know to mix the audio?