MuteController Code Error?

I am creating a simple script that will toggle the audio depending on if tab is pressed. Here is the code I have:

 using UnityEngine;
 using System.Collections;
 
 public class MuteController : MonoBehaviour {
 
     var muted = false;
 
     // Update is called once per frame
     public void Update()
      {
          if (Input.GetKey("tab") && (muted == false))
              AudioListener.volume = 0;
              muted == true;
          else if (Input.GetKey("tab") && (muted == true))
              AudioListener.volume = 5;
              muted == false;
      }
 }
 

But, when I save it and go back to Unity, it gives me this error:

 Assets/MuteController.cs(13,30): error CS0201: Only assignment, call, increment, decrement, and new object expressions can be used as statements.

Could anyone help me out? I've just started learning C# a few weeks back.

EDIT: I have even tried this code:

 using UnityEngine;
 using System.Collections;
 
 public class MuteController : MonoBehaviour {
 
     bool isMuted = false;
 
     // Update is called once per frame
     public void Update()
      {
          if (Input.GetKey("tab") && (isMuted == false))
              {
                  AudioListener.volume = 0;
                  isMuted = true;
              }
          if (Input.GetKey("tab") && (isMuted == true))
              {
                  AudioListener.volume = 1;
                  isMuted = false;
              }
      }
 }
 

After I tried this new code, it doesn't give me an error, but it doesn't toggle the audio at all. EDIT FINAL: I got it to work on the final code. I had to change the second if statement to else if. Works perfectly!