Device-to-device audio communication (think QR codes, but made out of sound) seems very underrated. Much better UX than QR codes or NFC because you don't have to worry about putting your phone in the exactly right place or pointing it in the right direction.

Anyone here with more detailed takes on this?

Should be outside of human (and dog) spectrum so it doesn't annoy anyone

But what if we want to hear cute AI-generated melodies that hide cryptographically encoded secrets?

When I was in Osaka for devcon, the ATM at the nearby 7-11 would play really fun music as it was dispensing cash. I remember this guy maybe in his 90s in front of me doing the little money dance as his withdrawal was being processed. What a fun and joyful vision for the future of payments 😁