Converting ordinary sound to a dolby 5.1 ??

[mobin.aca]

Hello everybody this is my first post on this site !! I have downloaded some video songs from the internet and i play them on my home theatre system with surround sound but the sound quality was poor and the rear speaker were not functioning !!
Can any of u tell me about some kind of software so that i can convert these videos and they become palyable on my dvd palyer with the 5.1 surround effect ??

Thanks