[MEncoder-users] problem re-encoding 5.1 audio ... channels get mixed up

[Phil P]

Hi,

I have a file with 5.1 audio that I want to re-encode as 5.1, basically 
so I can apply the "volnorm" filter and raise the sound level.

I use the following command:
mencoder <input-file> -ovc lavc -oac lavc -lavcopts 
acodec=ac3:abitrate=448:vbitrate=1350:vpass=1 -ffourcc DIVX -channels 6 
-af volnorm -o <output-file>

The command completes correctly but when I play the file on my 5.1 
speakers I find the channel outputs have been transposed ie. the 
original file plays fine but when playing my re-encoded file the centre 
channel appears to be front-right, front-right is now rear-right... etc...

As I'm not applying any channel filters I wouldn't have expected this to 
happen.

Does anyone know why this occurs?
Is it a common problem?
Is there any workaround?
Is there an alternative method to normalize the volume on the original 
stream?

I've searched the archive and couldn't locate a similar question.

Much thanks,
Phil.