When adding audio recording to a video surveillance system, there are legal and technical factors to consider.

Legal Issues

In the United States, the laws of most states require that there be adequate posting of a warning that audio surveillance in use.
Technical factors
The biggest challenge in recording audio with a video surveillance system is synchronization. For example, it is difficult to synchronize the audio input on a PC with a video stream coming from a channel. For this and other reasons, if audio recording is required then a specialized DVR is used that typically takes audio and video input on the same input channel.