If a music file isn't found, have the server send its MD5 hash (unique code/string made from the contents of a file) to the player and check it against all other music files. If it matches, use that file when asked to play the sound that wasn't found. If it doesn't, download it and use it. Problem solved; reduces problems from downloading the same things over and over again. (You could do this with the model files and textures as well: no more downloading from servers "Add-Ons/Vehicle_StuntPlane[001]/".)