edited @tresf, added checklist

The MP3 patents have finally expired. Since LAME is available on all platforms, LAME will be used and bundled to provide this functionality.

• Export song to mp3 via GUI (Implement #2000 ("Add mp3 encoding and/or decoding support") #3615)
• Export song to mp3 via CLI(Implement command line export for MP3 (#2000) #3641)
• Import tracks as mp3 via SampleTrack, AudioFileProcessor(Add MP3 import #6750)

The discussion on #1471 recently led to some talk over the possibility of supporting the mp3 file format (specifically in decoding samples, but it also has use as an export option).

For those that don't know, there are a variety of patents associated with encoding and decoding mp3's. In order to do either option, you must obtain a license. These patents are all scheduled to expire in the U.S. sometime before 2018 (the last one expires 30/Dec/2017), according to this source. Other sources (liftarn's comment) indicate that September 2015 is the month that mp3-related patents expire in the U.S. However, the patents have already expired in most other countries, and may have never applied to some countries.

LAME

Now, there is a non-commercial (LGPL-licensed) mp3 encoder/decoder, LAME. According to Wikipedia, distributing LAME in compiled form along with LMMS would likely be in violation of the US patents. A popular open source tool, Audacity, is able to provide mp3 encoding through LAME by detecting its presence on the system at runtime instead of shipping precompiled binaries. They explain the reasoning briefly at the top of this page. This appears to remove responsibility from the developers of Audacity over patent licensing, instead pushing that responsibility to the users that are affected (i.e. U.S. users who download LAME). Additionally, that same page says that Thomson (the patent enforcer) permits mp3 encoding for "personal" use, which a decent portion of LMMS users do.

So if we want mp3 support before the patents expire, we could go with a similar approach to Audacity if all the developers are comfortable with it. I sure am not a lawyer, so I can only judge based upon prior projects, of which Audacity is a relevant one to reference.

Gstreamer

Another option to supporting mp3's is through gstreamer. gstreamer is essentially a plugin-based audio routing library. As such, it provides plugins for both encoding and decoding a variety of audio formats, including mp3.

Historically, I think Gstreamer has been more linux-oriented, as it was adopted by the gnome project fairly early. But it claims to support OS X and Windows plus mobile.

I wish I knew more about this option. All I can say is that it seems like a more generalized approach to encoding and decoding that happens to support mp3's.

Anyways, feel free to discuss your opinions or concerns about mp3 support in lmms, whether you think it's worth adding now or best to wait for the patents to completely expire, and which implementation approach might be the best. Also, clarifications on the expiration dates or legal issues are much appreciated if you have that knowledge.