Just about everyone has the my music glitch including me lol.
Anyways i found some a code to fix that:
Mushroomy Kingdom (Above and Below) My Music Reset
4A000000 9017EC6C
14000000 00002714
14000004 00000432
14000008 00002723
1400000C 00000432
14000010 0000271D
14000014 00000432
14000018 00002715
1400001C 00000532
14000020 00002716
14000024 00240532
14000028 00002717
1400002C FFFF0532
E0000000 80008000
Boot brawl on Gecko OS or Riivolution go to My Music select Muschroomy Kingdom you'll see all the music bar thingy is reseted (the icon is in the middle of the music bars) then turn off the wii turn off the code and go back to My Music and...No freeze. You can edit your music anyway you want and no freezing will happen