Easiest way to fade sound in your unity project using scripting. Create AudioHelper.cs and paste the code below.

using System.Collections;
using UnityEngine;

public static class AudioHelper {

	public static IEnumerator FadeOut(AudioSource audioSource, float FadeTime) {
		float startVolume = audioSource.volume;
		while (audioSource.volume > 0) {
			audioSource.volume -= startVolume * Time.deltaTime / FadeTime;
			yield return null;

	public static IEnumerator FadeIn(AudioSource audioSource, float FadeTime) {
			audioSource.volume = 0f;
			while (audioSource.volume < 1) {
				audioSource.volume += Time.deltaTime / FadeTime;
				yield return null;

How to use in your project

Since this function is an IEnumerator it must be started as a coroutine. This example I get the audiosource from the gameobject the script is attached to. I set fadeTime to 2 seconds.

private AudioSource soudtrackAudioSource;
private float fadeTime = 2f;

void Start () {
  soudtrackAudioSource = GetComponent<AudioSource>();

StartCoroutine(AudioHelper.FadeIn(soudtrackAudioSource, fadeTime));
StartCoroutine(AudioHelper.FadeOut(soudtrackAudioSource, fadeTime));