I just tried it, never realized my laptop has a built in microphone.
I had to turn my speaker volume down so it wasn't always red. In the classroom you would have to get the students to talk at level you want and set your computer volume then.
If you have a smart phone or iPad, there are free apps that will display the noise level. I found this one on Pinterest with the suggestion that you connect it to a projector/SmartBoard so the students can monitor themselves. I wouldn't trust it to give an accurate decibel measurement, but maybe say kids need to keep it below a certain number.