Metal. The way it works, is that what makes a "song", is the melody and lyrics.
So, you want a vocalist to sing 'something' to your track.
in the end it you would not own any rights to your "song".
So, if I took your track and sang:
The pounding death inside my soul
will take you to the edge
as I chew upon your carcase
on a platter in my bed.
I would own that, and the melody that I used to sing it.
In other words, The Song.