soens
slartabartfast
Although you can request that SoundCloud remove your work from public access at any time, under 17 U.S. Code § 203 (b) (1):
"A derivative work prepared under authority of the grant before its termination may continue to be utilized under the terms of the grant after its termination, but this privilege does not extend to the preparation after the termination of other derivative works based upon the copyrighted work covered by the terminated grant."
...HUH???!? I have no idea what that even means!
OK. In even plainer English and with some background:
Congress is trying to protect two artists in this clause. The first artist is the original author/composer of a work or art. He is granted the sole right to make a "derivative work" i. e. to include it in a compilation like a best of CD, to change the words or music, to sample small portions to put in another song etc. This is an exclusive right not subject to a compulsory license, so only the original author can grant a license (the "grant" above) to anyone else to do any of those things, without which the second artist cannot make a derivative work without infringing the first artist's copyright.
The second artist buys a license from the first that enables him to make a derivative work of the original, or allows him to make derivative works for a period of time after which he must stop making new derivative works (the termination of the grant). The second artist alters his work from the original sufficiently or puts it in a creative context that allows him to claim his own copyright to the portions of the original work that are his unique contribution. He cannot claim any rights in portions of the work that were created by the original artist, but he has expended effort to create the derivative work for which he expects to be compensated. He then begins publishing the derivative work, which he created under license (the authority of the grant), and is making a money off it. The first artist decides that he wants to take back the license (terminate the grant) he sold to the second artist. If he could simply do that, he could stop the second artist from selling any more copies of the second artist's creation--the derivative work.
The section above protects the second artist from losing his work, by saying that the first artist cannot do a take back. Once the derivative work has been created it is out of the original artist's control, and the rights in the new portions of the derivative work belong to the second artist. the original artist can prevent the second artist from creating any new derivative works, by terminating his grant of a license, but any work that was created while the license is in effect belongs to the second artist exclusively.
In the context of the SoundCloud TOS, once you have uploaded your song, agreeing to the TOS grants users the right to make derivative works of your song for free if they find it there during the time when it is covered by the licensing provisions of the TOS. So if you decide to tell SoundCloud to take your song off the service, even assuming that such action would terminate a grant of the license, anyone who has already made a derivative work based on your song can do anything he wants with it until the copyright the user obtained by creating the derivative work expires, after which he can still do anything he wants because it has entered the public domain (which lasts forever).