title: Error with youtube_dl on Lambda 'ffprobe/avprobe and ffmpeg/avconv not found.'
body: <p>I want to get wav from youtube with Lambda and upload it to S3.
  Youtube_dl is created by layer and loaded into Lambda, and youtube-dl, ffmpeg, and ffprobe are placed in the bin of youtube_dl of layer, but the same error occurs.
  I want to resolve the following error.</p>
  <p>error</p>
  <pre><code>ffprobe/avprobe and ffmpeg/avconv not found.
  </code></pre>
  <p>Lambda</p>
  <pre><code>from __future__ import unicode_literals
  import youtube_dl
  import json
  import boto3
  import urllib.parse
  import requests
  import os
  
  def lambda_handler(event, context):
  
      ydl_opts= {
          'format': 'bestaudio/best',
          'postprocessors': [{
              'key': 'FFmpegExtractAudio',
              'preferredcodec': 'wav',
              'preferredquality': '192'
          }],
          'postprocessor_args': [
              '-ar', '16000'
          ],
          'prefer_ffmpeg': True,
          'outtmpl': '/tmp/%(title)s.%(ext)s',
          'keepvideo': True
      }
      
      with youtube_dl.YoutubeDL(ydl_opts) as ydl:
          ydl.download(['https://www.youtube.com/watch?v=NEDa3yGUWu0&amp;t=215s'])
  
      bucket = 'youtube-sounds-wav'
      key = '%(title)s.wav'
      tmpPath = '/tmp/'+key
      s3.meta.client.upload_file(tmpPath, bucket, key)    
      os.remove('/tmp/*')
  
      return
  </code></pre>