Ok, for the mp3 thing, just get an mp3 file, and make a folder in the base directory called music, keep making the directories like this:
  music/your_level/name_of_music.mp3 
 in radiant, just click a world spawn, hit "n" and type music for the key, and for the value type music/your_level/name_of_music.mp3 if it doesnt work, switch the slash marks around (/ and \)  
 for new textures, make a .jpg file, and then make sure its size is divisible by 2 (128X128, 256X256, or 512X512 etc...) if you dont make it divisable by 2, the level wont run at all. Put the texture in a directory called textures\your_map\texture.jpg  
 for cutscenes, start with this tutorial:  
 Kengo's Tutorials (
http://www.geocities.com/kengomaps/tutorials.html) 
 they are the best tutorials for camera movements. I dont think you need a .nav file... or you dont in jk2 (they could have done something weird in JA)