While the tech giant states in its policies that it may analyse audio recordings to improve translations, it does not make clear that humans may be listening in, according to Motherboard.
Microsoft says it gets permission from users before collecting and using their voice data.
The report claims to have obtained audio recordings which include intimate conversations between loved ones, while others were said to be heard talking about personal issues such as weight loss.
Workers can also listen to voice commands spoken to Microsoft’s virtual assistant Cortana, it is suggested.
“Microsoft collects voice data to provide and improve voice-enabled services like search, voice commands, dictation or translation services,” a Microsoft spokeswoman said.
“We strive to be transparent about our collection and use of voice data to ensure customers can make informed choices about when and how their voice data is used.
“We also put in place several procedures designed to prioritise users’ privacy before sharing this data with our vendors, including de-identifying data, requiring non-disclosure agreements with vendors and their employees, and requiring that vendors meet the high privacy standards set out in European law.
“We continue to review the way we handle voice data to ensure we make options as clear as possible to customers and provide strong privacy protections.”
The news comes amid increased pressure on tech firms to be more transparent about the use of human reviewers who listen to voice recordings, largely with the aim of improving voice recognition features.
Amazon has decided to give users of its Alexa virtual assistant the option to opt out of having humans analyse their audio, while Apple and Google have decided to suspend the practice.