In Boston there's a law requiring ALL public transport vechicles to make announcements, including busses.
If there's no automatic system in place, the driver has to do it on a mic, or in some of the MBTA's oldest busses, by shouting down the bus at each stop.
The new busses all had GPS-based systems that announced each major junction and stop the bus passed. They also had external speakers which announced info about the bus at the stop!
The on-train systems were audio-only but comparable to the Luas and the DART when it's system works.