SoundManager.cs

using UnityEngine;
using UnityEngine.UI;

public class SoundManager : MonoBehaviour
{
    public static SoundManager Instance { get; private set; }
    [SerializeField] Slider volumeSlider;
    public const float defaultVolume = 0.5f;


    void Start()
    {
        if (!PlayerPrefs.HasKey("soundVolume"))
            PlayerPrefs.SetFloat("soundVolume", defaultVolume);

        Load();
    }

    public void Awake()
    {
        //singleton
        if (Instance == null)
        {
            Instance = this;
            if (gameObject.IsRoot())
                DontDestroyOnLoad(gameObject);
        }
        else
        {
            Destroy(gameObject);
        }
    }

    private void OnDestroy()
    {
        Instance = null;
    }

    public void ChangeVolume()
    {
        AudioListener.volume = volumeSlider.value;
        Save();
    }

    private void Load()
    {
        volumeSlider.value = PlayerPrefs.GetFloat("soundVolume");
    }

    private void Save()
    {
        PlayerPrefs.SetFloat("soundVolume", volumeSlider.value);
    }
}