nostr:npub1tz8dj0rnhvfh0zdy7cr20t93gy70l64r7lrqj7sknqy6m9su3e3smjpgny Ohhh touch schreen keyboards... I dunno, it would be harder to do with sound, maybe not possible.. but a simple RF based solution that reads the signal passing through the phone could probably do it.

Either way im sure you could get that information though the approach may be a bit different is all.