Per this old article from 2016, yes: https://medium.com/waymo/sounds-of-the-self-driving-car-c26f...

The culture of using the horn differs a lot between countries. Would they use locale aware honk schemes?

And similarly are self driving cars listening? Will they detect emergency vehicles by the sirens, or even listen to other cars honking at them?

Emergency and police vehicles by sirens yes, but likely not other cars.

Clarification - the article you posted is about the Waymo technology, not the Uber technology. It's possible that Uber does not use those capabilities.