Audio Channel Mapping to 5.1

[maik]

Hello

I have a mpg-video with stereo sound.
Now I want to create a MP4 avi-video with 5.1 sound.
It was possible to ceate this video also with stereo sound.
I tryed to change the sound to 5.1 with the "Cannel mapping" in the "sound" tab.
Is it possible to switch form stereo to 5.1 with this option?
If yes, wich options I have to set?
Wich conditions I have to grant to change the sound to 5.1?

I always got the message "Failed load MEncoder. This is possible that the decoder is absent or it doesn´t support the specified file"

Thanks, for any help.

Maik