Audio playing when not colliding issue c#

I have a script that has a function that I need to both play audio when not colliding and stop playing audio when I do collide. But I don't know how to call my function in update, and I don't know how to use Stop, and I can't find out how anywhere in unity's scripting library. Please help!

using UnityEngine; using System.Collections;

public class Falling : MonoBehaviour { public AudioClip FallSound;

 private AudioSource Source;
 private float volmin = .5f;
 private float volmax = 1.0f;

 void Start () {
     Source = GetComponent<AudioSource>();
     gameObject.GetComponent<Rigidbody2D>().freezeRotation = true;
     transform.position = new Vector3 (0f, -0.7f, -1f);
 }

 void FallPlayerSound (Collision col){
     if (col.gameObject.tag == "Obstacle") {

             //Stop playing the sound? Somehow?

     } else {
         float vol = Random.Range (volmin, volmax);
         Source.PlayOneShot(FallSound, vol); 
     }
 }
 
 void FixedUpdate () {
     if (transform.position.y > 3.75) {
         Destroy(gameObject);
     }
     if (transform.position.y < -4) {
         Destroy(gameObject);
     }

             //I guess I should call FallPlayerSound here? But how? And with what arguments?

 }

}