The Musopen website requires all uploaders to "represent and warrant that content uploaded to the site is in the public domain" (see FAQ #4).
Musopen's own works ("Musopen Materials" and "Musopen Service", not user content or public domain content), are fully copyrighted and may not be used freely as stated in their Terms and Conditions of Use.
to share – to copy, distribute and transmit the work
to remix – to adapt the work
Under the following conditions:
attribution – You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
share alike – If you remix, transform, or build upon the material, you must distribute your contributions under the same or compatible license as the original.