const cp = require('child_process');

var ffmpeg = cp.spawn('ffmpeg', [
    '-i', 'in1.mp4',
    '-i', 'in2.mp3',
    '-map', '0:v',
    '-map', '1:a',
    '-c', 'copy',
    'pipe:1',

]);
ffmpeg.stdout.pipe('outputnew.mp4');