Apple and Google are temporarily stopping workers listening to voice recordings captured by smart speakers and virtual assistants.
It follows a Guardian report that third-party contractors used by Apple had heard people having sex and discussing private medical information.
Siri and other services can activate in error after wrongly picking up sounds they mishear as their “wake” words.
Apple said the move would affect users worldwide.
Google suspended the practice across the EU on 10 July but has only just confirmed the fact in public.
Amazon – which also uses staff to transcribe some recordings – has not commented.
Technology companies use staff to “grade” voice recordings to improve their virtual assistants’ accuracy rates in handling requests – and take steps to anonymise the source.
Google, for example, distorts the recording before it is listened to in order to disguise the user’s voice.
However, many members of the public were unaware of the practice until the Bloomberg news agency reported the fact earlier this year.
“We are committed to delivering a great Siri experience while protecting user privacy,” Apple said in a statement.
“While we conduct a thorough review, we are suspending Siri grading globally.”
The company added in the future users’ voice recordings would not be included in the grading process unless they had chosen to opt in.
Germany’s data protection commissioner in Hamburg has also launched an investigation into Google over the practice, with which the search company is cooperating.
Speech-recognising assistants were “highly risky” from a privacy point of view, the regulator said in a press release.
“The use of speech assistance systems must be transparent so that informed consent can be obtained from users,” added the commissioner, Johannes Caspar.
Google said it had already stopped transcribing voice recordings and would continue to do so for at least three months.
A spokeswoman for the company said Google was “in touch” with the Hamburg data protection commissioner.
“We don’t associate audio clips with user accounts during the review process, and only perform reviews for around 0.2% of all clips,” she added.
BBC News has asked Amazon whether it also plans to suspend speech monitoring.
In June, Dave Limp – the executive in charge of Alexa – said only a “tiny fraction of 1%” of voice recordings were ever listened to by humans.
But he conceded the company’s terms and conditions could have been clearer about the matter.