Alex教程每一P的教程原代码加上我自己的理解初步理解写的注释，可供学习Alex教程的人参考
此代码仅为较上一P有所改变的代码

【Unity教程】从0编程制作类银河恶魔城游戏_哔哩哔哩_bilibili​​

AreaSound.cs

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

public class AreaSound : MonoBehaviour
{
    [SerializeField] private int areaSoundIndex;

    private void OnTriggerEnter2D(Collider2D collision)
    {
        if(collision.GetComponent<Player>() != null)
        {
            AudioManager.instance.PlaySFX(areaSoundIndex, null);
        }
    }

    private void OnTriggerExit2D(Collider2D collision)
    {
        if(collision.GetComponent<Player>()!=null)
        {
            AudioManager.instance.StopSFXWithTime(areaSoundIndex);//退出区域后，声音缓慢减少
        }
    }
}

AudioManager.cs

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

public class AudioManager : MonoBehaviour
{
    public static AudioManager instance;

    [SerializeField] private float sfxMinimumDistance;
    [SerializeField] private AudioSource[] sfx;
    [SerializeField] private AudioSource[] bgm;

    public bool playBgm;
    private int bgmIndex;
    private bool canPlaySFX;//

    private void Awake()
    {
        if (instance != null)
        {
            Destroy(instance.gameObject);
        }
        else
            instance = this;

        Invoke("AllowSFX", 1f);//让游戏延迟一秒后才能发出声效
    }
    private void Update()
    {
        if (!playBgm)
            StopAllBGM();
        else
        {
            if (!bgm[bgmIndex].isPlaying)
                PlayBGM(bgmIndex);
        }
    }
    public void PlaySFX(int _sfxIndex,Transform _source)
    {
        //if (sfx[_sfxIndex].isPlaying)//防止出现多个相同物体同时发出声音
        //{
        //    return;
        //}
        if(canPlaySFX == false)
        {
            return;
        }

        if (_source != null && Vector2.Distance(PlayerManager.instance.player.transform.position, _source.position) > sfxMinimumDistance)//防止东西很远的情况下发出声音
            return;

        if(_sfxIndex < sfx.Length)
        {
            sfx[_sfxIndex].pitch = Random.Range(.85f, 1.1f);
            sfx[_sfxIndex].Play();
        }
    }
    public void StopSFX(int _sfxIndex)
    {
        sfx[_sfxIndex].Stop();
    }
    public void PlayRandomBGM()
    {
        bgmIndex = Random.Range(0, bgm.Length);
        PlayBGM(bgmIndex);
    }
    public void PlayBGM(int _bgmIndex)
    {
        bgmIndex = _bgmIndex;
        StopAllBGM();
        if (_bgmIndex < sfx.Length)
        {
            bgm[_bgmIndex].Play();
        }
    }
    public void StopAllBGM()
    {
        for(int i = 0; i < bgm.Length; i++)
        {
            bgm[i].Stop();
        }
    }
    private void AllowSFX()
    {
        canPlaySFX = true;
    }
    public void StopSFXWithTime(int _index)
    {
        StartCoroutine(DecreaseVolume(sfx[_index]));
    }
    private IEnumerator DecreaseVolume(AudioSource _audio)//退出区域后，声音缓慢减少
    {
        float defaultVolume = _audio.volume;

        while (_audio.volume > .1f)
        {
            _audio.volume -= _audio.volume * .2f;
            yield return new WaitForSeconds(.6f);
            if (_audio.volume <= .1f)
            {
                _audio.Stop();
                _audio.volume = defaultVolume;

                break;
            }
        }

    }
}

UI.cs

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

public class UI : MonoBehaviour,ISaveManager
{
    [Header("End screen")]
    [SerializeField] private UI_FadeScreen fadeScreen;
    [SerializeField] private GameObject endText;
    [SerializeField] private GameObject restartButton;
    [Space]

    [SerializeField] private GameObject characterUI;
    [SerializeField] private GameObject skillTreeUI;
    [SerializeField] private GameObject craftUI;
    [SerializeField] private GameObject optionsUI;
    [SerializeField] private GameObject inGameUI;

    public UI_itemTooltip itemToolTip;
    public UI_statToolTip statToopTip;
    public Ui_SkillToolTip skillToolTip;

    public UI_CraftWindow craftWindow;

    [SerializeField] private UI_VolumeSlider[] volumeSettings;

    public void Awake()
    {
        SwitchTo(skillTreeUI);//修复可能出现skill没法加载成功的bug

    }
    public void Start()
    {
        SwitchTo(inGameUI);
        itemToolTip.gameObject.SetActive(false);
        statToopTip.gameObject.SetActive(false);
    }

    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.C))
        {
            SwitchWithKeyTo(characterUI);
        }

        if (Input.GetKeyDown(KeyCode.B))
        {
            SwitchWithKeyTo(craftUI);
        }

        if (Input.GetKeyDown(KeyCode.K))
        {
            SwitchWithKeyTo(skillTreeUI);
        }

        if (Input.GetKeyDown(KeyCode.O))
        {
            SwitchWithKeyTo(optionsUI);
        }
    }

    public void SwitchTo(GameObject _menu)//切换窗口函数
    {

        for (int i = 0; i < transform.childCount; i++)
        {
            bool fadeScreen = transform.GetChild(i).GetComponent<UI_FadeScreen>() != null;//保证存在淡入淡出效果的函数时才会为真，才会使darkScreen保持存在

            if (!fadeScreen)
                transform.GetChild(i).gameObject.SetActive(false);
        }

        if (_menu != null)
        {
            AudioManager.instance.PlaySFX(7,null);
            _menu.SetActive(true);
        }
    }

    public void SwitchWithKeyTo(GameObject _menu)//键盘切换窗口函数
    {
        if (_menu != null && _menu.activeSelf)//通过判断是否传入mune和mune是否激活来决定使设置为可视或不可使
        {
            _menu.SetActive(false);
            CheckForInGameUI();
            return;
        }
        SwitchTo(_menu);
    }

    private void CheckForInGameUI()//当其他UI不在时自动切换值InGameUI函数
    {
        for (int i = 0; i < transform.childCount; i++)
        {
            if (transform.GetChild(i).gameObject.activeSelf && transform.GetChild(i).GetComponent<UI_FadeScreen>() == null) //修复InGameUI在fadeScreen打开后，没法存在的问题
                return;
        }

        SwitchTo(inGameUI);
    }
     
    public void SwitchOnEndScreen()//死亡综合效果函数
    {
        SwitchTo(null);
        fadeScreen.FadeOut();
        StartCoroutine(EndScreenCorutine());
    }

    IEnumerator EndScreenCorutine()//死亡显示文本函数
    {
        yield return new WaitForSeconds(1);
        endText.SetActive(true);
        yield return new WaitForSeconds(1.5f);
        restartButton.SetActive(true);
    }

    public void RestartGameButton()//场景重开函数
    {
        GameManager.instance.RestratScene();//调用GameManager的重开函数
    }

    public void LoadData(GameData _data)
    {
        foreach(KeyValuePair<string,float> pair in _data.volumeSettings)
        {
            foreach(UI_VolumeSlider item in volumeSettings)
            {
                if(item.parametr == pair.Key)
                {
                    item.LoadSlider(pair.Value);
                }    
            }
        }
    }

    public void SaveData(ref GameData _data)
    {
        _data.volumeSettings.Clear();
        foreach(UI_VolumeSlider item in volumeSettings)
        {
            _data.volumeSettings.Add(item.parametr, item.slider.value);
        }
    }
}