Issue: I was not getting any sound when the HDMI cable is connected from the laptop (LG Gram 15) to an external monitor (Samsung Syncmaster PX2370).

Additional info: The monitor is not connected to any external speakers.

Fix:

Control panel -> View by: Small icons -> Sound -> Playback
-> Select a playback device below to modify its settings:
-> select "Speakers; Conexant HD Audio" -> Set Default -> OK.

Before I changed it to the above setting, it was set to “SyncMaster; Intel(R) Display Audio” which was not working.