オーディオのフェードイン・フェードアウト：DOTween 使用（ブログ公開用）

SoundManager.cs

using System;
using System.Collections;
using UnityEngine;
using UnityEngine.Audio;
using DG.Tweening;

// Singleton として持ち続ける
public class SoundManager : MonoBehaviour {
    static SoundManager instance;

    public AudioMixerGroup BGMMixerGroup;
    public AudioMixerGroup SEMixerGroup;

    GameObject soundObject;
    AudioMixer mixer;
    AudioSource bgmSource;

    static string bgmVolumeParam = "BGMVolume";
    static string seVolumeParam = "SEVolume";
    static float defaultVolume = 0.8f; // 0 dB

    void Awake() {
        if (instance == null) {
            instance = this;
            mixer = BGMMixerGroup.audioMixer;
            createSoundObject();
            DontDestroyOnLoad(this.gameObject);
        } else {
            Destroy(this.gameObject);
        }
    }

    void createSoundObject() {
        soundObject = new GameObject("SoundObject");
        GameObject.DontDestroyOnLoad(soundObject);
        bgmSource = soundObject.AddComponent<AudioSource>();
    }

    public static void PlayBGM(string path) {
        instance.playBGM(path);
    }

    void playBGM(string path) {
        DOTween.Kill(instance.mixer);
        SetBGMMixerVolume(defaultVolume);

        var clip = Resources.Load<AudioClip>(path);
        bgmSource.clip = clip;
        bgmSource.outputAudioMixerGroup = BGMMixerGroup;
        bgmSource.loop = true;
        bgmSource.Play();
        Resources.UnloadUnusedAssets();
    }

    public static void SetBGMMixerVolume(float volume = 0.8f) {
        instance.mixer.SetFloat(bgmVolumeParam, volumeToDecibel(volume));
    }

    public static void SetSEMixerVolume(float volume = 0.8f) {
        instance.mixer.SetFloat(seVolumeParam, volumeToDecibel(volume));
    }

    public static void FadeInBGM(float fadeDuration = 2.0f) {
        instance.mixer.DOSetFloat(bgmVolumeParam, volumeToDecibel(defaultVolume), fadeDuration)
            .From(volumeToDecibel(0))
            .SetEase(Ease.InCubic);
    }

    public static void FadeOutBGM(float fadeDuration = 2.0f) {
        instance.mixer.DOSetFloat(bgmVolumeParam, volumeToDecibel(0), fadeDuration)
            .From(volumeToDecibel(defaultVolume))
            .SetEase(Ease.InSine);
    }

    static float volumeToDecibel(float volume) {
        return volume * 100 - 80; // -80dB ~ +20dB
    }
}