Oh idk, maybe because it sometimes takes you 10 minutes to respond, so we don't have to be staring at the chat screen like a **, have the message make a sound once you finally get around to replying
Is it not a feature in the chat-software or browser you can activate yourself ?
But as Airbnb response time is extremely slow, i guess a chime will not help, as your device will fall asleep or go in ''standby"mode before receiving a reply (probaby will be "the case is closed").....