Stingers, I've figured out that music code. For the X's, you just put the number of the song that you want to replace, going in increments of 8 from 0-750 (that's a hex value, not actual 750). The offset chart is to help you find where to start when counting songs. Notice how FD starts at 028? That's because Battlefield's 5 songs are the 5 values before that (0000, 0008, 0010, 0018, 0020).
So if you wanted to replace the 3rd song on Yoshi Island, you'd go to the offset, 240, and go up 2 (times 8) which gives us 0250 (because 2*8 is 16, which is 10 in hex), and put that where the XXXX is. Then you go pick a song from the longer list, and put that 4 digit number where the YYYY is. The Dark World, for example, is 2736. So now, in My Music, under Yoshi Island, whatever the 3rd song in the list was would now be The Dark World, and will stay that way in memory Until you write another code and change it again. Make any more sense?