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Hundreds of smartphone apps are monitoring users through their microphones
(www.the-independent.com)
This is a most excellent place for technology news and articles.
Well these days Android asks for more permissions so I guess it would prevent it in many cases by preventing access to the microphone for apps where you don't want to allow it...
Android also shows an indicator when any app is accessing the microphone or camera now.
https://nvd.nist.gov/vuln/detail/CVE-2023-21083
There are probably other exploits around that as well
While these bugs happen, they are fixed very quickly, and wouldn't be used by any marketing agency. Just keep your phone updated.