I found a link on how to make your own 5.1 surround DVD-A :
http://forum.doom9.org/showthread.php?t=83752

There are many ways to make given in the link above, but I personally used this link:
http://forum.doom9.org/showthread.php?t=106500

I'm not audio-savvy so to say and I just followed each step mentioned there. It's really simple, but quite tedious. The end result is unbelievable. I just test it in the car and what a difference it is.