This follows several other companies who have revealed to listen to their customers' voice recordings, with various reasons; most common one being improving the voice recognition on said software.
Companies including Facebook, apple and amazon have stated that they are listening to recordings gathered using their various programs- from Facebook's messenger app, to Siri to Amazon's Alexa and Google smart microphone.
Usually, the listeners were human contractors who were hired to listen to the conversation and bring it back to the parent company.