Ok first of all u need to have a decent connection since everyone dl ur sounds before u join into the game, Second i use this alot not sound clips from films but to use in a clan fight for quick chats, as in, GoGoGo, or Cover me! etc,
Ok now to explain how to do it. First of all convert the music you want to OGG or any other sound format ofp works with i think its .wss then in ur user folder, where ur user information is create a folder called sound, them put all the sounds in there, try not have them to large cause it takes ages for everyone to dl them.
To activate them in game, u use the radio. Press 0 to get up the menu then Custom 9, then simpley pick what one u want by the name of the sound file. The name of the sound file is also the text that shows in game.
Hope that helps