basiclly i don't know how to do this, i've seen a video of someone with custom music on starcraft how did he do this?

err just saying, it could have been a fake (-> music played in backgorund, sc2 music disabled), but you can add music to a sc2 map as a loop, high quality
but directly change the background music of a race is not possible in the beta, at least I never saw an option about it, so I guess without modding the files it's not possible.
HOWEVER, someone else should response and correct me if I'm wrong.

You'd need to make a .sc2mod file and ATM you can only have 20mb mods on battle.net2, so it isn't really worth it.