Voice to Text on iOS10?
The iPhone does have a native voice-to-text feature so one can dictate emails and texts. You could open Notes, for example, and then enable the voice-to-text by touching the microphone on the keyboard (not the one next to the text-input). It's to the left of the spacebar. 

Depending on what you want to use it for, the following apps may be useful to you. 

Ava - This might do what you want it to. It helps with verbal conversations. 

Speak2See - This also might work for you. Someone else could speak into your phone, and it shows nice large words in text on your screen. 

Braci - Probably not what you're after. It analyzes sounds like doorbells and alerts your phone.

RogerVoice - This probably isn't quite what you're looking for, but it's still interesting. It was originally a KickStarter and is now on the App Store.