From the website of libsndfile, which is what SFML uses to read audio files, it's not very clear if FLAC is supported or not in versions < 1.0.18 (I think SFML 1.6 uses libsndfile 1.0.17).
The history says FLAC support was added in version 1.0.12, but in the Features section the author says "From version 1.0.18, libsndfile also reads and writes FLAC and Ogg/Vorbis".