Type out UI text one character at a time

UITextTypeWriter.cs

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

// attach to UI Text component (with the full text already there)

public class UITextTypeWriter.cs : MonoBehaviour 
{

	Text txt;
	string story;

	void Awake () 
	{
		txt = GetComponent<Text> ();
		story = txt.text;
		txt.text = "";

		// TODO: add optional delay when to start
		StartCoroutine (PlayText());
	}

	IEnumerator PlayText()
	{
		foreach (char c in story) 
		{
			txt.text += c;
			yield return new WaitForSeconds (0.125f);
		}
	}

}

Here is a simplier approach, without audio

using System.Collections;
using System.Collections.Generic;
using TMPro;
using UnityEngine;
using UnityEngine.SceneManagement;

[RequireComponent(typeof(TextMeshProUGUI))]
public class ScrollText : MonoBehaviour
{
    public string GameSceneName;
    public float InitialDelay = 0.1f;
    public float CharWait = 0.5f;
    public float CommaWait = 0.3f;
    public float PeriodWait = 0.5f;

    TextMeshProUGUI textMesh;
    string text;

    // Start is called before the first frame update
    void Start()
    {
        textMesh = GetComponent<TextMeshProUGUI>();
        text = textMesh.text;
        textMesh.text = "";
        StartCoroutine(TypeText());
    }

    IEnumerator TypeText()
	{
        yield return new WaitForSeconds(InitialDelay);
        foreach (char c in text)
		{
			textMesh.text += c;
            yield return new WaitForSeconds(CharWait);
			if (c == ',')
				yield return new WaitForSeconds(CommaWait);
			if (c == '.')
				yield return new WaitForSeconds(PeriodWait);
		}
        SceneManager.LoadScene(GameSceneName);
    }
}