[Kanegasi]

That's not the readycheck sound path, that's the levelup sound path, which is also the same sound as guild invites. The correct path is sound/interface/readycheck.ogg unless you want your custom sound to play whenever you level up or get a guild invite.

Since you're using the second code, you might as well just put your file in the addon folder you just installed with the code and use that path. The default ready check sound will still be replaced, since the code is looking for that id specifically, and the levelup sound will be untouched. When you move the file, make sure to quit and restart WoW.

I'm glad the code works in any case.