Can't play any sounds. What's wrong?

For the first code snippet you are trying to call audio.Play(); on a destroyed object.For calling audio on destroying object use something like this.Use another script like SoundManager.cs on a gameobject named SoundManager for playing sounds.

 //SoundManager.cs script
 public class SoundManager:MonoBehaviour
 {
   void Start()
   {
     //Create the audio source 
 
     //Load the audio clip
   }
 
   //The public function we will use to call to play the sound
   public void PlayJumpSound()
   {
     //Play the sound
     audio.play();
   }
 }

Now use your trigger scrpit like this

 void OnTriggerEnter2D(Collider2D other)
 {
   if(other.tag=="Player"
   {
     //Play the audio
     GameObject.Find("SoundManager").GetComponent<SoundManager>().PlayJumpSound();
     //Incriment the jump attempts
     jumper.jumpAttempts++;
     //Destroy the gameobject
     Destroy(gameObject);
   }
 }

The update function will call once per frame.That is if your FPS is 60,the Update() method will call 60 times a second.using audio.Play() directly in this method will keep on calling this method which will not give you the proper result.Use a boolean to make sure the function will only get called once

 bool IsSoundPlayed=false;
 
 if (Input.GetKeyDown("escape"))
 {
   paused = !paused;
 
   if(!IsSoundPlayed)
   {
     myAudio.Play();
     IsSoundPlayed=true;
   }
 }
 
 if(paused)
 {
   Time.timeScale = 0;
   //State to false
   IsSoundPlayed=false;
 }
 else
 {
   Time.timeScale = 1;
   //State to false
   IsSoundPlayed=false;
 }