I googled quite a bit to find patches for ffmpeg that will allow conversion from 5.1 audio channels into stereo. This is necessary if you want to convert all sorts of videos into FLVs with mp3 audio channel (eg. if you operate a video streaming service), since mp3 supports only mono and stereo channels.
Here're the patches:
Unfortunately neither of these two are as sophisticated as the algorithms in various media players (like Quicktime, VLC, Mplayer, etc.).
P.S.: I'm aware of the fact that Flash Player supports AAC audio codec (and H.264 video codec), but only since v9.0.115 (see the respective article
in Adobe's Wiki). There're still many computers (at the moment of writing) that run a lower version of the Flash Player. So depending on your goal (on percentage of supported clients), you might want to choose to encode your FLVs with H.264 and AAC (which supports 5.1 audio).