If only your spoken messages are not arriving it is likely you did not allow Braintoss to access your microphone on first use (a dialogue question). To adjust this go to Settings - Privacy - Microphone and check if Braintoss is enabled.
If this does not help it might be that you domain that does not accept MP3 files (mails with MP3s are then discarder by the security filter). You can test if this is the case by removing the MP3 files from the Braintoss emails for your address. To do this send a braintoss text not with the text:
set audioattachment off
If the emails then do arrive, you probably want to leave it as it is. Often the transcription is clear enough as a reminder. But if you receive an emtpy Braintoss mail and can not remember what you said, you can still check your history in the app settings. It's not pretty but we have no other solution for now. What you could try is ask your network administrator to make an exception on MP3s for our domain (braintoss.com).
See How do I change the format of the email? for more options.