Google finally released Duo the other day nearly three months after announcing it. The pitch for Duo at I/O was that it makes video chat really quick and easy—it makes video chat simple, but what about audio calls? A Googler says that's coming too.
This news comes by way of Amit Fulay, Google's product lead for communication. Fulay was responding to a query on G+ when he stated plainly that audio-only calls are coming to the app "soon." Of course, that's a "Google soon." It could mean in a week or later this year. It makes some sense when you consider that Google is refocusing Hangouts, which has voice calls, as a business product.
Audio-only calls in Duo could be handy in situations when you have limited bandwidth or your cellular signal is too sketchy for a regular call. Or maybe you just don't want to make yourself look presentable. Whatever your reason, it'll be an option at some point in the not too distant future.