MixerVolumeSlider.cs

using UnityEngine;
using UnityEngine.Audio;
using UnityEngine.UI;

namespace Citrus.Sound {
    [RequireComponent(typeof(Slider))]
    public class MixerVolumeSlider : MonoBehaviour{

        [SerializeField] private AudioMixer audioMixer;
        [SerializeField] private string volumeExposeParameter;

        private Slider slider;

        private void Awake()
        {
            slider = GetComponent<Slider>();
        }

        private void Start()
        {
            slider.minValue = 0f;
            slider.maxValue = 1f;
            slider.onValueChanged.AddListener(SetVolume);
        }

        /// <summary>
        /// ボリュームを変更する
        /// </summary>
        /// <param name="value">[0, 1]</param>
        private void SetVolume(float value)
        {
            float db;
            if (value < 0.0001f)
            {
                db = -80f;
            }
            else
            {
                //  db変換
                db = 20 * Mathf.Log10(Mathf.Clamp(value, 0.0001f, 1.0f));
                db = Mathf.Clamp(db, -80f, 0f);
            }
            audioMixer.SetFloat(volumeExposeParameter, db);
        }
    }
}

ミキサーのボリュームのパラメータをExposeしないと使えない