OmniFocus does have the ability to record voice notes, using the "Record Audio" button in the details screen of a new action, if that helps. I think we have a feature request for a bigger button and/or some sort of 'new item/new photo/new voice note' picker with bigger buttons filed; if that would help, be sure to email the support ninjas
and let them know. They can vote that up on your behalf.
It's possible that Apple could add Text-to-speech capabilities to the iPhone OS which OmniFocus could then take advantage of. But it would be extremely difficult for us to write a feature like that all by ourselves, and you'd only be able to use it in OmniFocus.
Letting Apple know this is something you'd like to see added, by using this form
, means you'd get the ability to use it in OmniFocus, Mail, and other iPhone applications as well. Thanks!