Prueba esto:
brew remove libogg flac sox
brew update
brew doctor
brew install libogg flac
brew install sox
Esto debería recompilar sox con libogg y flac presentes, y luego encontrará el soporte FLAC.
~$ brew info sox
sox: stable 14.4.1
http://sox.sourceforge.net/
/usr/local/Cellar/sox/14.4.1_1 (15 files, 2.1M) *
Built from source
From: https://github.com/Homebrew/homebrew/blob/master/Library/Formula/sox.rb
==> Dependencies
Build: pkg-config ✔
Required: libpng ✔, mad ✔
Optional: opencore-amr ✘, libvorbis ✔, flac ✔, libsndfile ✔, libao ✘, lame ✔
==> Options
--with-flac
Build with flac support
--with-lame
Build with lame support
--with-libao
Build with libao support
--with-libsndfile
Build with libsndfile support
--with-libvorbis
Build with libvorbis support
--with-opencore-amr
Build with opencore-amr support