Won't the eventual bleed or fade of the tattoo significantly change the "sound" recognized by the app? What happens when they no longer update the app or it's removed from the app store or there isn't an app store anymore?
I saw this post about tattoos as they age. Something to consider when you're relying on a really detailed tattoo to hold up over time.
http://www.boredpanda.com/tattoo-aging-before-after/