vcpkg pulls in its own FLAC build, I guess when running in debug mode they also use a debug build for FLAC, thus you're getting those information on the terminal. If you switch to release mode then it should also be using FLAC in release mode and the information should not be displayed anymore.
When using the FLAC builts shipped with SFML, you won't see them, since we've built FLAC in release mode.
So I guess if you don't want those messages in debug mode, you'll probably have to check with vcpkg to not build FLAC in debug mode or similar.