Add an intro audio file to a list of audio files in PHP using SoX.

audio-edit.php

<?php
/* Make sure you have SoX installed.
 * * Ubuntu install: `sudo apt-get install sox`
 * * Mac install (using Homebrew): `brew install sox`
 */
$audio_intro = '/var/www/wp-content/uploads/audio-intro.mp3';
$audio_output_dir = '/var/www/wp-content/uploads/audio-output/';
$audio_files = array(
  0 => array(
    'id' => 1,
    'title' => 'Audio File 1',
    'file_path' => '/var/www/wp-content/uploads/audio-originals/1'
  ),
  1 => array(
    'id' => 2,
    'title' => 'Audio File 2',
    'file_path' => '/var/www/wp-content/uploads/audio-originals/2'
  )
);
foreach ($audio_files as $audio_file) {
  $input = $audio_file['file_path'];
  $output = $audio_output_dir.$audio_file['id'].'.mp3';
  $command = 'sox '$audio_intro.' '.$input.' '.$output;
  shell_exec($command);
  $new_audio_url = 'http://example.com/wp-content/uploads/improved/'.$audio_file['id'].'.mp3';
}
?>