I need a video editing software that can edit .mp4?

I have a lot of videos on my computer that are .mp4. I would like to edit them by putting audio over most of the videos and keep the file as a .mp4 since it is the lowest size. Right now I am using windows movie maker and that can only do .wmv files that take so long to convert. So is there any software that you can edit .mp4 videos by making the real background noise quiet and adding your over it? Also I need the screen ratio to be 16:9 and to at least make it 720p