test/Assets/PerfectWorld/Scripts/Sound/AudioManager.cs

using System.Collections;
using UnityEngine;

public class AudioManager : MonoBehaviour
{
    public static AudioManager Instance;
    public AudioSource bgmSource;

    void Awake()
    {
        if (Instance == null)
        {
            Instance = this;
            DontDestroyOnLoad(gameObject);
        }
        else Destroy(gameObject);
    }

    public void PlayBGM(AudioClip clip, float fadeTime = 1f)
    {
        StartCoroutine(FadeInBGM(clip, fadeTime));
    }
    public void StopBGM(float fadeTime = 1f)
    {
        StartCoroutine(FadeOut(fadeTime));
    }
    

    IEnumerator FadeInBGM(AudioClip clip, float fadeTime)
    {
        if (bgmSource.isPlaying)
            yield return StartCoroutine(FadeOut(fadeTime));

        bgmSource.clip = clip;
        bgmSource.Play();

        float t = 0f;
        while (t < fadeTime)
        {
            t += Time.deltaTime;
            bgmSource.volume = Mathf.Lerp(0, 1, t / fadeTime);
            yield return null;
        }
    }

    IEnumerator FadeOut(float fadeTime)
    {
        float startVol = bgmSource.volume;
        float t = 0f;
        while (t < fadeTime)
        {
            t += Time.deltaTime;
            bgmSource.volume = Mathf.Lerp(startVol, 0, t / fadeTime);
            yield return null;
        }
        bgmSource.Stop();
    }
}