Open your brsar in Smashbox and check the group after the group you replaced (like if you replaced Mario's voice, then you should check Link's as snd_group_chara_link is under snd_group_chara_mario). If none of the elements contained by it can be opened (or Smashbox displayed errors after opening the brsar with it), the pack you've inserted was wrong. Also, make sure the region matches (NTSC packs can't be used with PAL and vice versa).
I did this and it replaced the sounds successfully but i still face the black screen after i click Launch.