app.get('/download', (req, res) => {

    var config = {
        host: 'someHost',
        port: '22',
        username: 'user',
        password: 'root'
    }
    var fileName = "hello.wav"; //File Name
    // Connect to server
    sftp.connect(config).then(() => {
        // Get the file as a readable stream
        sftp.get(fileName).then((data) => {
            res.setHeader('Content-disposition', 'attachment; filename=' + fileName);
            // Set headers to download file
            res.setHeader('Content-Type', 'application/audio/wav');
            // pipe stream
            data.pipe(res);
        });
    });
});