It does work as 64-bit library on a 64-bit architecture - however, getting it to work isn't all that easy. (word of warning though: the resulting executable will not start on a 32-bit architecture - unlike using a 32-bit library on a 64-bit architecture).
Basically you'll have to compile it for 64-bit (with for instance Visual Studio's 64-bit command prompt so that CMake adds the /machine:x64 to the appropriate compilers/linkers).
However, this isn't enough - the extlib stuff like jpeg, glew etc provided in the subversion repo are 32-bit only. There's another thread on this forum with appropriate ones for 64-bit compilation, you have to move the old 32-bit ones away and add the 64-bit ones in place to get those to work.
After that, what remains is libsndfile-1.dll, the one provided in the subversion repo is 32-bit only, but you can download the 64-bit one from their website.
I have all the files required at home (at work now) if that would be of any help. It would be nice if they were included by default into the subversion repo however.
i.e:
extlib/msvc/x64
so that linking to the proper libs when compiling sfml is as easy as changing the library path.