There is more going on here than literacy. Even if you are illiterate, if you tell someone to pay 10 rupees you could see if 10 rupees made it to your account.
Do those vendors not have smartphones? That's what I am suspecting. If they did, an app could announce it and you wouldn't need the external speaker.
The speakers seem to be directly connected to the cellular network. And rented for ridiculously small fees.