Just out of curiosity, how do you know, that your voice is being captured without your knowledge? Did you notice something strange or suspicious with your device?
Unfortunately, there is not a lot that you can do if your Android phone is not rooted. Usually, there are no hidden applications, unless it is a system one (Which spyware generally doesn't go that far). And you can't see system level applications without a rooted phone, so there is also that issue. Spyware generally is disguised as another more legitimate application or attached to one of the running applications as a service, you do have an option to stop and kill any processes.
I would strongly recommend disconnecting your phone from the internet, restarting the device and checking your running applications, kill and uninstall any suspicious ones that you will find. Additionally, you can try scanning with Malwarebytes if you are worried about it. Let me know how it goes.