LoopbackMicrophone.cs

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

[RequireComponent(typeof(AudioSource))]
public class LoopbackMicrophone : MonoBehaviour {
    public int prepareBufferSize = 1024;

    AudioClip clip;
    AudioClip playbackClip;
    AudioSource source;
    bool preparing = true;
    int head = 0;
    const int samplingFrequency = 44100;
    const int lengthSeconds = 1;
    float[] processBuffer = new float[256];
    float[] microphoneBuffer = new float[lengthSeconds * samplingFrequency];

    void Start() {
        clip = Microphone.Start(null, true, lengthSeconds, samplingFrequency);
        playbackClip = AudioClip.Create("LoopbackMicrophone", lengthSeconds * samplingFrequency, 1, samplingFrequency, false);

        source = GetComponent<AudioSource>();
        source.clip = playbackClip;
        source.loop = true;
    }

    void Update() {
        var position = Microphone.GetPosition(null);
        if (position < 0 || head == position) {
            return;
        }
        clip.GetData(microphoneBuffer, 0);
        playbackClip.SetData(microphoneBuffer, 0);
        if (preparing && position >= prepareBufferSize) {
            preparing = false;
            source.Play();
        }
    }
}