Yes, it's discussed before.

http://wiki.freeswitch.org/wiki/Channel_Variables#RECORD_STEREO

set that var to false before you record.


2009/10/10 Jason White <jason@jasonjgw.net>
Nagalenoj <nagalenoj@gmail.com> wrote:
>
> No, When I do voicemail_inject and check through voicemail, it is not playing
> the file instead reporting the following error.
>
> 'Stereo is currently not supported, please downsample to mono.'

This has been discussed on the list before. I think the solution was to use
sox to convert the files to mono. You might need a script to do this before
injecting them into the voicemail.