exporting AC3 5.1 audio

I’m using Audacity 2.0.3 on Windows 7 Professional x86.
I have to change time (from fps 23.976 to fps 25) in AC3 5.1 (6 channels) audio file- Well, after opening file(FFmpeg compatible), and changing time everything is ok but when I go to “Export” and selecting AC3 format and bitrate I got message :“The…will be saved as mono channel” .
Why it is not saved in 6 channels, same as source ? Am I missing some steps ? Or do I need some additional libraries ?

Thanks in advance

To export 5.1 you need to enable “Use custom mix” in “Edit > Preferences > Import/Export”.
See here for more information: http://manual.audacityteam.org/o/man/advanced_mixing_options.html