PlumberUltimateAds/Assets/ButtonSoundAdder.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class ButtonSoundAdder : MonoBehaviour
{
    public Button[] AllButtons;
    void Start()
    {
        if (AllButtons.Length == 0)
        {
            AllButtons = FindObjectsOfType<Button>();
            for (int i = 0; i < AllButtons.Length; i++)
            {
                AllButtons[i].onClick.AddListener(() =>
                {
                    Sound.instance.PlayButton(Sound.Button.Back);
                });
            }
        }
    }
}