Bit confused by your question.
You say that want to send audio text message i.e. an audio SMS (really an MMS) but then you say that you are in the email app.
To send an audio text message open the Messages app, then either enter the recipient's name or number first (or do it later), or tap on the green + sign in the type text message line and then tap on the Record Audio icon.
The Record Audio app will open. Tap on the red spot and being recording your message, tap on the white/gray square too stop recording and then select Done.
Your recording will now appear in the Message app where you started (remember if you didn't do it before to enter the recipient's name or number) and then send the message.
To ensure that MMS can be sent from your phone, check that the Data Connection setting in Settings > Wireless and Networks > more > Data Connection is enabled. If not you can only send/receive SMS messages and not send/receive MMS messages.