Is it legal to record people’s voices at work place?

At my job I noticed some black stick thing on top of a shelf and thought it was just some random decor, then later found out by a co worker that it’s a microphone. Im not that smart when it comes to laws, but isn’t that illegal here in Illinois? I get video recordings, but audio? I’m pretty sure no one is aware of it and probably won’t notice it since of how well hidden it is