iPhone mic transmit to earpiece?

I am deaf in one ear and have trouble conversing in a noisy restaurant. I am looking for a way to place my iPhone 5 on the table in front of my partner so that the sound picked up by its built-in microphone is transmitted to an earphone I would wear. Suggestions ?

You could try the Mimi Music app, which has that feature. The App Store page lists iOS 9 as the minimum requirement, so there's a good chance it will run.

iOS12 supports this out of the box, and apparently it even works with AirPods. Apple has a support article with all the details online.

But with AirPods it’s even easier:

Enable it in Control Center

• Settings -> Control Center -> Customize Controls
• Tap (+) on Hearing

Now access Control Center by swiping up from the bottom of the screen and you will see an icon with an Ear

How to Use Live Listen Feature in iOS 12

• Pair and put on your Airpods in to your Ear
• Access Control Center
• Tap the Hearing Sign
• Tap On & Done