How do I put music on my WMM videos?

Well, I like to make videos on WMM and post them on YouTube. I think that my videos are too boring without music, so once I tried it and YouTube disabled the audio, and i deleted the video. So, I decided to remake the whole video except with pitching the audio. I got a song editor and did that, but Windows Movie Maker doesn't accept it. It says that I don't have "the required codec" installed on my computer. I tried downloading the codec, but it didn't work. The only music I can put in there is the WAVE versions that I got from songs on iTunes, but I can't post it on YouTube. Please help!!??!?!?!?!?