Who identified the copyright "infringement" is an important issue. Soundcloud typically uses a robot detection system on uploaded material to determine if it "matches" something in their database. If Soundcloud itself claims the infringement, then it should be cleared up by filing a dispute with SoundCloud. There are also for-profit services that use similar technology to dredge online sites for work that their clients pay them to find and either demand payment for or issue a DMCA takedown notice. If a copyright "owner" issues a takedown notice, then most likely you will need to respond to that in a specific legal format, in which case Soundcloud will have to re-instate your work unless the DMCA issuer objects, in which case he will have to file suit for copyright infringement within a short period of time, or lose his right to take the work down. With a work that is clearly in the public domain, and this would not include original ancient work that has been rearranged by a modern author, then the only right in serious question would be the mechanical rights to the recording, which does not apply in this case.