I don't know how other smart speakers (eg. Google Home/Alexa) do this, but the Yandex Station smart speaker uses this for initial wifi setup. You enter the Wifi

I don't know how other smart speakers (eg. Google Home/Alexa) do this, but the Yandex Station smart speaker uses this for initial wifi setup. You enter the Wifi password on your phone in their app, then your phone plays a series of sounds in which the SSID and the password are encoded, the speaker listens to that and connects to the network.