file_list = Dir['original/*.wav']
file_list.each_with_index do |original_wav, index|
  basename = File.basename(original_wav)
  target_wav = "processed/#{basename}"
  gain = case basename
         when /^piano/; 4
         else; 0
         end
  puts "[#{index + 1}/#{file_list.length}] #{basename} (#{gain})"
  system "sox", original_wav, target_wav, "gain", "#{gain}" or raise "Cannot convert"
end