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From the developers: The "joint stereo/ stereo" options refer to the "channel mode" command that Audacity sends to LAME (which encodes the MP3).Lame only uses that command if the audio is being exported with two channels.If Audacity is sending single channel mono for encoding, then LAME ignores the channel mode instruction.The settings in the MP3 options control all subsequent MP3 exports until the user changes the settings. The "channel mode" instruction can't be "greyed out" when exporting a mono track because if the channel mode option is not set then LAME will not know what to do if the next file to be exported is a stereo track. The MP3 options do not only apply to the current exported file, they apply to all exported MP3 files.