It’s a tricky situation due to the patent issues with MP3 - there are few libs with compatible licensing (LAME, for instance, is LGPL for encoding, but GPL for decoding - d’oh). Can’t wait until it expires - I guess it’s 2017 in the US and a bit sooner elsewhere.
I think it should be appropriate to use a LGPL DLL linked against a UE4 plugin, but I’m no lawyer or LGPL specialist. I think it works because the plugin code is open, though. Not sure.
In any case, I’ll try to provide MP3 support through an LGPL library and leave it at that (I’m not super concerned about having the MP3 people come after me, honestly…worst case I have to take the plugin down).