CODE EXAMPLE FOR CSHARP

Attach Mixer to Audio Source via script

using System;
using UnityEngine.Audio;
using UnityEngine;

public class AudioManager : MonoBehaviour
{
    public Sound[] sounds;

    public static AudioManager instance;

    // use for initialization
    private void Awake()
    {
        // to avoid multiple audio manager in some scene
        // and we don't want music to restart every time we change scene.
        if (instance == null)
            instance = this;
        else
        {
            Destroy(gameObject);
            return;
        }

        DontDestroyOnLoad(gameObject);

        foreach (Sound s in sounds)
        {
            s.source = gameObject.AddComponent<AudioSource>();
            s.source.clip = s.clip;
            s.source.outputAudioMixerGroup = s.mixer;

            s.source.volume = s.volume;
            s.source.pitch = s.pitch;
            s.source.loop = s.loop;
        }
    }

    private void Start()
    {
        Play("background");
    }

    public void Play (string name)
    {
        //FindObjectOfType<AudioManager>().Play("name");

        Sound s = Array.Find(sounds, sound => sound.name == name);

        if (s == null)
        {
            Debug.LogError("Sound: " + name + "not found!");
            return;
        }
        s.source.Play();
    }
}


// heres the other public Sound code
using UnityEngine.Audio;
using UnityEngine;

[System.Serializable]
public class Sound
{
    public string name;

    public AudioClip clip;

    public AudioMixerGroup mixer;

    [Range(0f, 1f)]
    public float volume;
    [Range(.1f, 3f)]
    public float pitch;

    public bool loop;

    [HideInInspector]
    public AudioSource source;
}