I was able to convert the video at that URL (the Maroon 5 song)...
So if the code is working for me, and I was able to convert a video that you are not able to convert, then I can only assume the problem is with your server setup...
If it was working for you before, then your server setup must have changed recently. I would:
1) Turn error handling on in the code (comment out ini_set('display_errors',0)
2) Check your server error logs
3) Make sure FFmpeg is functional within your server setup
4) Download a fresh copy of FFmpeg
5) Make sure that you can execute commands on the server with PHP's exec()
Also I saw a comment somewhere else in this thread about which PHP version can run this code. Since my code uses PHP 5 OOP, it should be run on PHP 5 or higher.