The reason that Audacity’s Libraries Preferences is tied to the recommended LAME and FFmpeg is that other versions of those libraries usually don’t have enough “symbols” compiled in, so the exported files are written without proper length information, with metadata missing and similar problems.
You can export using arbitrary versions of LAME or FFmpeg by choosing “external program” when you export then pointing Audacity to the compiled binary of your choice - that is, to the LAME.EXE or FFmpeg.EXE on Windows (or to the “LAME” or “FFmpeg” file on Mac), not to the DLL or DYLIB file.
For LAME, try http://www.rarewares.org/mp3-lame-bundle.php (you may need to rename binaries as “LAME” on Mac).
For FFmpeg, use the “static” builds on http://ffmpeg.zeranoe.com/builds/ for Windows or http://ffmpegmac.net/ or http://www.evermeet.cx/ffmpeg/ for Mac. There are no guarantees with latest FFmpeg that all will work correctly in Audacity.
For help exporting using (external program), see http://manual.audacityteam.org/o/man/exporting_to_an_external_program.html .
You can always build any version of LAME or FFmpeg you want from their sources.
Gale