To play your own .mp3 files in the menus, just go to your launch box folder then go to music then go to a folder called background I believe, and then put your desired music files in that folder and they will play when inside of bigbox. For sounds, I believe that if you name any .mp3 albeit short, a second of two long file, with the appropriate names for them, I don't know the four file names off the top of my head, then you should be able to have your own custom menu sounds.
Hope that helped!