From the comments:
I would agree that it's not Facebook Messenger's intention to record audio or take a photo without being initiated (eg. taking/adding a pic to a text msg) but once you give permission for the app to do so automatically, what's to stop a hacker or other app from doing so? We have too much blind faith...
I have a solution for that:
As someone who worked on the Messenger team at one point I can say with confidence that articles like this one are nothing but naive scare-mongering.
Facebook has messed up a few times in the past with privacy related issues so whenever anything related to terms and conditions comes up people love to skew what they read more to the negative side.
If these kinds of permissions didn't exist you'd never be able to take photos, record voice or do almost anything really in the way of messaging or talking with your friends using the app. These kinds of permissions are just par for the course.
It asks me to access the mic when trying to take a picture which is pretty annoying since I now always have to double take pictures.
Curious of others thoughts on this?