I set to true because brian said it can play stereo files but no lucky for me.

2009/10/12 Jason White <jason@jasonjgw.net>
Seven Du <dujinfang@gmail.com> wrote:
> originate {ignore_early_media=true,RECORD_STEREO=true}sofia/gateway/xx/xx
> &bridge(sofia/gateway/yy/yy)

Shouldn't that be record_stereo=false for mono recording?