UI button OnClick - cannot play a disabled audiosource

[Sinusgamma]

I try to build my first, simple singleton soundmanager, which hold reference for all sound effects trough some scenes. I have a SoundManager object with two Audio Sources for background music and mouse click sound. In the SoundManager script I have a function (buttonTrigger) which plays the sound, when the button is clicked. I added this function to the UI button On Click() settings.
Problem: when I try to click I get this message: cannot play a disabled audiosource.
But this isn’t disabled, I don’t know what is the problem.

Any idea?
Thanks

using UnityEngine;
using System.Collections;

public class SoundManager : MonoBehaviour {

    public static SoundManager Instance;

    public AudioSource menuMusic;
    public AudioSource clickSound;

    void Awake()
    {
        // there is one single public static instance of one class
        if (Instance == null)
        {
            DontDestroyOnLoad(gameObject);
            Instance = this;
        }
        else if (Instance != this)
        {
            Destroy(gameObject);
        }
    }


	// Use this for initialization
	void Start () {
        StartMenuMusic();
    }


    public void StartMenuMusic()
    {
        menuMusic.loop = true;
        menuMusic.Play();
    }

    
    public void buttonTrigger()
    {
        clickSound.Play();
    }
}

[Sinusgamma]

I tried to solve this for some hours, but after posting this question I have the solution:

    public void buttonTrigger()
    {
        SoundManager.Instance.clickSound.Play();
    }

So I think I have to use the CLASSNAME.Instance in a singleton, to be able to communicate with the outside world

Earlier I tried this, but maybe I didn’t save the project.