What should i do for audio for my public prank?

Hi, I'm new at this. For right now i don't have all the nice new wired microphones or anything, but i would like to shoot my first public prank video tomorrow with what i have. I have an iPhone 5s. I'm a little bit stuck on what to do about audio. The person filming will be behind a shopping cart or something. Is there an app or something i can do for audio? Please reply as anything is better than nothing.

Bomb threat

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