code: function(cb) {
          if (process.env.FFMPEG_PATH) {
            fs.exists(process.env.FFMPEG_PATH, function(exists) {
              if (exists) {
                cb(null, process.env.FFMPEG_PATH);
              } else {
                cb(null, '');
              }
            });
          } else {
            cb(null, '');
          }
        }
docs: Try FFMPEG_PATH