Changing the Soundtracks on games? NFS: Underground

[Tango87]

I read once in this forum about changing the soundtrack in a game by using PlashFXP and taking out the games tracks and then adding your tracks and burning the new game to a disc. But when I looked in the NFS: Underground folders there were no tracks to be found... I know they are there because there are like 15 of tehm in the game but they suck so I want to enter my own. Someone please helkp me out with this if they have an answer.