I think Avi worked on something similar:
http://wiki.fusionpbx.com/index.php?title=Voicemail#Transcribing_Voicemails_with_google_speech_API
From: freeswitch-users-bounces@lists.freeswitch.org [mailto:freeswitch-users-bounces@lists.freeswitch.org] On Behalf Of Michel Daggelinckx
Sent: Thursday, January 12, 2012 3:43 PM
To: FreeSWITCH Users Help
Subject: [Freeswitch-users] speech to text engine
found this one on thepiaf forum.
mightbe usefull for a freeswitch module