How can I video a thunderstorm and hear the sound?

You probably won't be able to. At least, not they way you want.

For a very long time now, devices that use automatic leveling do their best to adjust the input volume level, based on the average decibel reading at the moment. They adjust the sensitivity up or down, depending on how much quieter or louder the ambient sound becomes.

However, there are no automatic systems that can adjust fast enough for a clap of thunder that loud. It's so loud compared to what it was recording earlier that the levels are way off the scale, and the sound gets clipped.

You'll notice on the opposite end, in very quiet areas, that the playback has a lot of hiss. That's the device turning the sensitivity up as much as possible waiting to pick something up. As soon as someone speaks, the background hiss goes away since the device has quickly turned the sensitivity down.

you will need to go where there is enough ambient noise from the storm where the phone can pick it up on the Mic. Keep in mind your iPhone is not waterproof or even water resistant and should not be used where it's going to be exposed to liquids.

The level of the thunderclap may not have been detected by the mic or may not be reproducible. Your range of hearing and the phones range of detecting or reproducing audio are very different, so is the camera which can detect ranges of light your eyes cant and vice versa.