This is probably a somewhat advanced level project, but for those who might enjoy a challenge, all the steps are laid out for you.

In a previous article I published a solution to convert Asterisk voicemail attachments from WAV to MP3 on the fly. This is done by catching the mails sent by Asterisk just before they are passed to sendmail.

I recently got the idea from Daniel Dainty to add Voice Recognition feature at the same time as mp3 encoding.

After testing different voice recognition engines, I realized that the Google Speech Recognition API is by far superior to any other solution available under Linux (Sphinx, …).

This article will explain an approach to add voice recognition to Asterisk voicemail using the services of Google Speech Recognition API.

Full article here:
Asterisk – Voicemail with Speech Recognition using Google API (Bernaerts family site)