If you have highly secured areas, you may need an audio/video intercom system to see and hear from people requesting access. If you just need audio recognition, then an audio only intercom will do the trick. Already have a security system that needs better audio pick up? Install additional microphones and you will be good to go.
2MCCTV.com has whatever you need.
Related Article: 3 Things to Know About CCTV Systems with Built-In Microphones