This simple ruby script will allow searching for and downloading video files from Youtube (as mp4 files) and then converting them to mp3 audio files, suitable for uploading to Soundcloud or a web server.

search_youtube_to_mp3.rb

require 'rubygems'
require 'youtube_search'  # https://github.com/grosser/youtube_search
require 'youtube_dl'      # https://github.com/tonic20/youtube_dl

FFMPEG = "/usr/local/bin/ffmpeg"

res = YoutubeSearch.search('music|song|musicians|performance|instruments', :author => 'TEDxTalks', :per_page => 50, 'orderby' => 'viewCount')
res.each do |r|
  puts "https://www.youtube.com/watch?v=#{r['video_id']} <#{r['title']}>"
  youtube = YoutubeDl::YoutubeVideo.new("http://www.youtube.com/watch?v=#{r['video_id']}", :location => './video')
  unless File.exist?("./audio/#{r['video_id']}.mp3")
    puts youtube.download_video
    `#{FFMPEG} -i ./video/#{r['video_id']}.mp4 ./audio/#{r['video_id']}.mp3`
  end
end