When I try to record a sound for my sprite in Scratch, nothing happens. I click on the microphone to record a new sound, then click on the circle to start recording, click on "allow" when that warning pops up, but then, nothing happens. My microphone, which is just the built-in one that came with my laptop, works in other applications. I'm using Google Chrome 43.0.2357.81 m on an Asus SonicMaster laptop with a Windows 8.1 operating system. I have the same problem when using Firefox.
You need to allow access to microphone in camera in two places before you can record with Scratch. Look in the upper right corner of Chrome and you may see a camera with a red x next to your favorites star. Click on that icon, and you should get a dialog that ask if you want to allow camera and microphone access. Select "Always allow scratch.mit.edu to access your camera and microphone" in this dialog and that should fix your problem.