Few things to check:
1. Are you able to hear audio in playback recordings via a web browser or iVMS 4200?
2. Via a web browser, navigate to
Configuration >> Storage >> Schedule Settings >> Advanced and ensure "Record Audio" is ticked. Try again.
3. Use either
Hikvision's own software, VSPlayer or another media player
such as MPC-HC as VLC often has issues playing audio from Hikvision cameras using the default audio codec.