SoundDelayAlternative.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class SoundDelayRepeat : MonoBehaviour 
{
  private AudioSource audioSource;
  private float lastPlay;
  public float firstTimeDelay;
  public float timeInbetweenSound;
	
	void Start ()
	{
    audioSource = GetComponent<AudioSource>();
    StartCoroutine( PlayLoop() );
	}

  IEnumerator PlayLoop () 
  {
    yield return new WaitForSeconds(firstTimeDelay);
    audioSource.Play();
    while(true){
      yield return new WaitForSeconds(timeInbetweenSound);
      audioSource.Play();
    }
  }
}

SoundDelayRepeat.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class SoundDelayRepeat : MonoBehaviour {

  private AudioSource audioSource;
  private float lastPlay;
  public float firstTimeDelay;
  public float timeInbetweenSound;
	
  void Start ()
  {
    audioSource = GetComponent<AudioSource>();
    lastPlay = firstTimeDelay - timeInbetweenSound;
	}

	void Update () 
	{
    if (Time.time > lastPlay + timeInbetweenSound) {
      timeInbetweenSound = Time.time;
      audioSource.Play();
      lastPlay = Time.time;
    }
  }
}

reworked with differing initial delay