News site Bloomberg highlighted the topic after speaking to Amazon staff who “reviewed” Alexa recordings.
All three companies say voice recordings are occasionally reviewed to improve speech recognition.
But the reaction to the Bloomberg article suggests many customers are unaware that humans may be listening.
The news site said it had spoken to seven people who reviewed audio from Amazon Echo smart speakers and the Alexa service.
Reviewers typically transcribed and annotated voice clips to help improve Amazon’s speech recognition systems.
Amazon’s voice recordings are associated with an account number, the customer’s first name and the serial number of the Echo device used.
Some of the reviewers told Bloomberg that they shared amusing voice clips with one another in an internal chat room.
They also described hearing distressing clips such as a potential sexual assault. However, they were told by colleagues that it was not Amazon’s job to intervene.