To enable synchronous playback of video and audio, click the icon in the upper-right corner of the frame. After this the icon looks like this and audio is played back synchronously with video in the archive playback mode.

To disable synchronous playback of video and audio, click the  icon. The icon will look like this , and video is played back with no audio. 

Absence of the  icon means that the camera is not configured for recording audio to the archive.

For the information on configuring synchronous playback of video and audio, see Configuring audio signals playback using the video monitor.