I just upgraded my LAME encoder to 3.96. To my surprise, the encoded MP3 files came out being in mono rather than (joint-)stereo. How come? As a frontend for LAME, I use the brilliant EAC. In EAC you can specify extra commandline option to pass to LAME, and in my case, that would be --r3mix. But this option is no longer recognised by LAME and the best it can make of it is -m. Hence a mono mp3 file. Obviously changing the commandline options to something more appropriate restored my joint-stereo and the quality settings.
Hope this will save someone from pulling hairs.