How to record audio for a prank in public?

Here is what I have to work with an iPhone, a spy pen with an HD camera that records 7 hours of video and audio, an HD Canon camera and a regular digital underwater camera.
I was thinking about using the iPhone voice memo then syncing the audio when i edit. I only have one shot so it needs to be nearly perfect. I don't want to buy anything

Pranks are the worst. Don't do pranks.

I suspect the voice memo produces a lower quality than you'd get by recording video and then just using the audio. Try experimenting and see which is best. Make sure the phone isn't rubbing against anything - the noise will drown out everything else.

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