I've finally ported the Master Sword Mod to 1.6.4. However, I couldn't port it to 1.7.2 because it's relatively new and there is currently no Forge for it.
  The sound system for 1.6.4 is kinda messed up for me, so the installation method changed a bit. It's better to have the installation change than not have the mod at all over a mere sound bug :P
  I've added a few new features in this new Master Sword Mod version; be sure to check it out!
  Just so you know in advance, the new Goddess's Harp is a little buggy; but it won't crash your Minecraft client.
  Well, see you again on the 1.7.2 update, or whenever I need to make a new post.

P.S
 If you know how to register sounds to Minecraft 1.6.4 with Forge and keep the mod in a .zip file to remain in the mods folder, please comment on this post. Much help would be appreciated!