http://blog.ragingflame.co.za/2013/5/31/using-nodejs-to-join-audio-files

currentfile = './files/' + clips.shift() + '.mp3';
stream = fs.createReadStream(currentfile);

把排好序的数组，逐个shift出来，读入到流

stream.pipe(dhh, {end: false});
    stream.on("end", function() {
        console.log(currentfile + ' appended');
        main();        
    });

当读完时，再去执行main，再次shift，读入到流。一直到

if (!clips.length) {
        dhh.end("Done");
        return;
    }

明白了？