Beginners troubles with audiosource

I'm still new to learning Unity and seem to have a problem.. I would like to change an audiosource's clip depending on the button that is pressed and play it.. it works when I use

 public void onPlaySound()
     {
        AudioSource WhateverSound = GetComponent<AudioSource>();
         WhateverSound.Play();
     }

but when I want to extend the scope of AudioSource so every function can access it I keep getting "is never assigned and will always have a value of null" and "nullreferenceException"..

I used the Unity Scripting documentation and tried:

 public class ExampleClass : MonoBehaviour {
     public AudioClip impact;
     AudioSource WhateverSound;
     
     void Start() {
         WhateverSound= GetComponent<AudioSource>();
     }
     
     void OnPlaySound() {
         WhateverSound.PlayOneShot(impact, 0.7F);
     }
 }

but I keep getting those errors.. I checked google and if found I needed to use the "new" keyword like

 AudioSource WhateverSound = new AudioSource();

but didn't work either..

I'm sure it's a stupid mistake, but I can't figure it out..