Dolby Narrate for iOS lets you add voiceover to video

I recently mailed a rather large check to a friend of mine named Susan. As a sort of joke, I also shot a short video, showing the stamped envelope with Susan's address followed by my hand putting the envelope into the mailbox. I wanted to show her that the check was literally in the mail.
The video captured a lot of the surrounding street noise, so I went home, where it's more quiet, and used Dolby's awesome new Narrate iOS app to remove the original soundtrack and attach own narratio