I don't think enabling downloads automatically waives copyright protection, but I also think the artist should remember to mark all works with the relevant copyright note. For me, that would be the word "Copyright" and/or the symbol © followed by the year and my name. Adding "All rights reserved" might also be a good idea. SoundCloud has an option that allows the poster to indicate what type of license s/he wants to apply to each song s/he uploads, if I remember right. It's early, I'm sleepy, so my brain is not functioning at full power....
I have wondered about the legal consequences of forgetting to mark a work with the copyright information, because I don't always remember. I think this whole issue of rights, licensing, copyrights is confused these days because of the internet and easy availability (and share-ability) of anything.
I believe the original writer should have control over what happens to anything s/he writes, but I think there is also a danger of copyright claims going too far. Like, if someone uses a phrase that is actually common in the spoken language and then tries to sue another artist for using the same phrase later on.