According to them, it seems that while combing through Snapchat’s code, they found mention of video and audio calls, as well as something called audio notes, which presumably lets you send short audio messages to yourself or to others. Messengers such as WhatsApp and WeChat have similar features where you can record your reply and send it to the other person.
This seems to be especially popular in China where it saves the user a lot of time from having to type out Chinese characters, so presumably this is a similar feature for Snapchat (although we are just speculating). This would also be a considerably smaller file compared to sending a video with audio, making it more efficient.
In any case no word on when these features are expected to make their way to Snapchat, but presumably it will be in the next update, so we guess we’ll just have to wait and see when that update is released.