If you're considering adding an audio component to your video surveillance system, you'll ant to carefully research the audio surveillance laws in your state to avoid potential lawsuits and/or criminal charges. Recording conversations can be a violation of privacy, so it's crucial to not only research -- but also understand your state's laws.
Let's take a look at New York's laws. In New York, all parties (or persons) must be informed of and consent to being recorded in a private conversation before a recording can take place. However, conversations occurring in the public sphere, i.e. in a park or walking on the street can technically be picked up on audio and video without the need for consent because of the limited expectation of privacy in these places.
Be sure to also speak with an attorney in your state about any lingering questions you have concerning audio recording. States where audio surveillance is permitted often require signs informing all parties that their conversations are being recorded.