You can change the BGM music, for example Harbour music, and ingame battle music. Thats all. Gun sounds and such are changeable, but i'm not sure how to do it manually. You can find sound packs if you look though.**
If you wish to have a music file changed go to your SD EnterNet Folder -> SOUND and in their replace a mp3 file of your choice with the exact same txt.
Example: you have "Battle.cxf" replace that with your own music file. but make sure it is still called "Battle.cxf"
**Note** Back up all your files incase it does not work, or you wish to go back to default.
Best be to keep it the same, but if you want you can change it. (I have mine changed.)
** Thank You for finding that Koala44202