A Commercial MP3 Encoder/Decoder to use with Sox/Audacity

Question

In recent days i've been looking into a way to mix together MP3 files at the point of upload to a website, which is hosted on a linux machine.

So far, thanks to the community, i've learned that using SoX over the command line is probably the most efficient way to actually produce a mixed file from two sources. I have also discovered that Audacity may also be useable for the purpose.

Unfortunately, both of these programs being free - they do not support the MP3 format due to legal issues around patents and licensing. I know that support can be added with LAME, however this still remains illegal because LAME is also free and is not licensed by the patent owners.

So I'm looking for a way to add MP3 encoder/decoder support to Sox, Audacity or a similar command-line enabled program, so that it can be used legally within a commercial website. Searches have once again come up blank, does anyone know of a solution please?