This is an actual attack surface: https://www.researchgate.net/publication/284949922_Keyboard_...

It doesn’t need to be unique, it just needs to leak enough information to decrease the search space enough to where brute force (or other methods) can kick in.

Each key will produce a different sound even if it's just a touch screen surface keyboard due to being in different positions on the surface and having a relative position to the microphone - it may be more difficult and require a higher quality microphone.

Once you isolate and cluster all the key sounds you end up with a simple substitution cipher that you can crack in seconds.