Simple static Class for AudioSource fade In and Out

AudioFader.cs

using System.Collections;
using UnityEngine;

public static class Fade
{
    public static IEnumerator In (AudioSource audioSource, float fadeTime)
    {
        const float startTime = 0f;
        var currentTime = 0f;
        audioSource.Play();
        while ((int)audioSource.volume != 1)
        {
            audioSource.volume = Mathf.InverseLerp(startTime, fadeTime, currentTime);
            currentTime += Time.deltaTime;
            yield return null;
        }
    }
    
    public static IEnumerator Out (AudioSource audioSource, float fadeTime)
    {
        const float startTime = 0f;
        var currentTime = 0f;
        while (audioSource.volume != 0f)
        {
            audioSource.volume = Mathf.InverseLerp(fadeTime, startTime, currentTime);
            currentTime += Time.deltaTime;
            yield return null;
        }

        audioSource.Pause(); // or stop, depending what do you want to do
    }
}