I think that the error message "SoX was compiled without MP3 encoding support" is misleading. It can convert from mp3 but not to mp3. If I run the command "sox x.mp3 y.mp3 tempo 1.5" it fails but the command "sox x.mp3 y.ogg tempo 1.5" works fine. So this issue is just related to writing mp3 files.
I think that the error message "SoX was compiled without MP3 encoding support" is misleading. It can convert from mp3 but not to mp3. If I run the command "sox x.mp3 y.mp3 tempo 1.5" it fails but the command "sox x.mp3 y.ogg tempo 1.5" works fine. So this issue is just related to writing mp3 files.