SimpleLoopback.cs

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

public class SimpleLoopback : MonoBehaviour {
    public string[] DeviceNames = new string[2];
    public int prepareBufferSize = 1024;

    AudioSource source;
    AudioClip[] microphoneClips = new AudioClip[2];
    AudioClip playbackClip;
    bool preparing = true;
    float[][] microphoneBuffers = new float[2][];
    float[] playbackBuffer = new float[44100 * 2];

    void Start() {
        for (var channel = 0; channel < microphoneClips.Length; channel++) {
            microphoneClips[channel] = Microphone.Start(DeviceNames[channel], true, 1, 44100);
            microphoneBuffers[channel] = new float[44100];
        }
        playbackClip = AudioClip.Create("SimpleLoopback", 44100, 2, 44100, false);
        source = GetComponent<AudioSource>();
        source.clip = playbackClip;
        source.loop = true;
    }

    void Update() {
        foreach (var device in DeviceNames) {
            if (Microphone.GetPosition(device) < 0) {
                return;
            }
        }

        for (var channel = 0; channel < microphoneClips.Length; channel++) {
            microphoneClips[channel].GetData(microphoneBuffers[channel], 0);

            for (var sampleIndex = 0; sampleIndex < 44100; sampleIndex++) {
                playbackBuffer[sampleIndex * 2 + channel] = microphoneBuffers[channel][sampleIndex];
            }
        }

        playbackClip.SetData(playbackBuffer, 0);

        if (preparing) {
            var prepared = true;
            foreach (var device in DeviceNames) {
                if (Microphone.GetPosition(device) < prepareBufferSize) {
                    prepared = false;
                }
            }
            if (prepared) {
                preparing = false;
                source.Play();
            }
        }
    }
}