Fade Audio

Hi guys, How can I do for make a fade at "Motore2" when i press A?

Sorry for my bad english!

 using UnityEngine;
 using System.Collections;
  
 public class Audio : MonoBehaviour
 {
     public AudioClip Motore1;
     public AudioClip Motore2;
     float soundPitch = 0;
     float maxSpeed = 2;
     float minSpeed = 0.2f;
  
  
  
     void Start()
     {
        if (!audio.playOnAwake) audio.Play();
     }
  
  
     // Update is called once per frame
     void Update () 
     {
        soundPitch = Mathf.Clamp(soundPitch, minSpeed, maxSpeed);
  
  
        if (Input.GetKey(KeyCode.A)) 
        {
          audio.clip = Motore1;
          soundPitch+= 0.09f;
          audio.pitch = (soundPitch);
  
        }
  
  
  
        if (Input.GetKey (KeyCode.D)) 
        {
          audio.clip = Motore2;
          soundPitch+= 0.09f;
          audio.pitch = (soundPitch);
        }
  
  
        if (Input.GetKey (KeyCode.S)) 
        {
          soundPitch-= 0.09f;
          audio.pitch = (soundPitch);
        }
  
     }
  
  
 }