On my Debian system, this happens unless libsndfile-dev is also installed apparently because Debian's libsndfile package doesn't install the .so files; only libsndfile-dev adds those and libltdl's load-library-at-runtime mechanism doesn't look for .so.3 files, only .so files among the extensions it tests for.
However, the fix to the bug report seems to say debian had disabled flac and mp3 hoping that falling back to sndfile to do them would work; I wouldn't do that as it the ways they drive libflac etc differs, sometime in way that can give different results so Debian sox would behave differently from standard SoX. If that is indeed the way it has been fixed, all well and good.
