FFMPEG and AVCONV amix audio files lowers a volume

Question

I am using this command

    foreach( $records as $record ) {
      if( $inputs == 0 ) {
          $meta_tags .= "1";
      } else {
          $meta_tags .= ":0";
      }

      $inputs++;

      $files .= ' -i ' . $path . $record;
}

$meta_tags .= "]";
$shell_command = 'ffmpeg '. $files .' -filter_complex "'. $meta_tags .'amix=inputs='. $inputs .':duration=longest" output.wav';

Everything goes well but when the mix is done, it lowers a volume. Can you please help me? I have tried avconv too.