Hey,
We had some stuff copyrighted a few years ago, all we did was find a good entertainment lawyer, signed a few documents, gave him a copy of our stuff, and we were good to go.
We were considering the 'post it in the mail' approach, but I don't know how legal that it...if you go with a lawyer, it gets documented and recorded somewhere. If you're serious about this, get a lawyer, it's what all the professional side of the industry are doing for good reason.
If your stuff has samples in it, and YOU OWN THE LICENCE OR RIGHTS TO THE SAMPLE, you should be good to go. But don't go sampling some old Jame Brown record, use it in a track, and try to copyright that...they'll come down on you HARD if you try to pass it off as your own.
You can copyright something even if it doesn't have lyrics, my music is all techno with no lyrics.
Take care,
Nick