I have the following script, which attached to an object with audio source, un-mutes the audio upon collision with FPPlayer.

How should I modify the script to mute the audio after a second collision and mute/un-mute it on every next (like a on/off button upon colliding)?

 using UnityEngine;
 // make sure the game object you attach it to has an AudioSource on it
 // create a script that you can attach to a game object
 public class SetVolumeOnCollision : MonoBehaviour {
     // the volume to set when collided; can be set in the Editor; 
     // not necessarily needed, but it will make the component more versatile
     [SerializeField] private float volumeOnCollision = 1;
     // the tag to which it should react; also not essential, but nice to have
     // make sure you set the tag of your colliding object if you do use it!
     [SerializeField] private string tagToReactTo = "Player";
     // the audio source you want to control
     private AudioSource audioSource;
     // this is called in the first moment, even before sounds start to play
     private void Awake () {
         // get the audio source to control that is on this game object
         audioSource = GetComponent<AudioSource>();
         // set volume to 0, muting it; you can also do this in the Editor
         audioSource.volume = 0;
         // make sure it wil start to play; this as well can be set in the Editor
         audioSource.playOnAwake = true;
     // this function is called on the frame when something collides with this game object
     private void OnCollisionEnter (Collision collision) {
         // check the tag; remove this if you don't want it
         if (collision.collider.CompareTag(tagToReactTo))
             // set the desired voluem; just replace it with 1 if you don't want to use desired volume
             audioSource.volume = volumeOnCollision;

Add a boolean member variable to the class, which records the current muteState;

bool muteState=false;

Also, add a float to record the originalVolume of the audio source, before you mute it.

float originalVolume;

Now, in OnCollisionEnter, you can Set the volume Based upon this:

if( ! muteState) //if not muted, mute it
   originalVolume= audioSource.volume ;
   audioSource.volume =volumeOnCollision; //muted  
else //if already muted, UN-mute it
   audioSource.volume = originalVolume; 

Then toggle the muteState like so:

muteState= ! muteState;