I have a reason for wanting to do this. I think mp3s encoded in 3.99.3 sounds better to me.
I’m running Audacity 2.3.0 on Linux Mint 18.3 based on Ubuntu 16.04 with Lame 3.99.5+repack1-9build1 (xenial) but I would like to try running Lame 3.99.3 instead. I thought maybe if I could run Audacity 2.3.0 as a portable application. But I did a search for running it as a portable application in Linux. I didn’t find any instructions for doing this. Instead I found 2.2.2 https://www.audacityteam.org/download/source/ from source. It would be nice if I could make it run as a separate application that I could put in 3.99.3 like you can in Windows.
If you can tell me how to downgrade my lame to 3.99.3 that would be something I could try.
If you can tell me how to run audacity as a portable application in Linux that is something I could try.
Better than Lame 3.99.5?
To see what has changed between versions, look to the changelog: LAME Changelog
I don’t see anything that would give a change in encoding quality, though I do see some bug fixes. (Audacity does not use LAME for decoding).
Creating a real “portable” version for Linux is not simple because by default, Audacity is built to uses shared libraries. You would have to ensure that all dependencies were included in your portable version.
To just run a separate, non-installed version, with it’s own settings, independent of any installed version, you can just build Audacity from the source code (instructions: https://github.com/audacity/audacity/blob/master/linux/build.txt), then add an empty folder called “Portable Settings” in the same location and the “audacity” binary.
A much simpler way to use an arbitrary version of LAME for exporting from Audacity, is to use Audacity’s command-line exporter - the “external program” option: https://manual.audacityteam.org/man/exporting_to_an_external_program.html