Joint-stereo is the default mode for stereo files with VBR when -v is more
than 4 or fixed bitrates of 160kbs or less. At higher fixed bitrates or higher
VBR settings, the default is stereo.
Normal stereo mode.
In this mode, the encoder will make use of a correlation between both channels
in order to achieve higher compression. This will enhance the quality of constant
bitrate recordings, and reduce the size of variable bitrate recordings.