You configure the audio output options via Control Panel -> Sounds -> "Playback" tab - see here for an example. Setting the HDMI option as the default should be all that's required.
I couldn't find anything on the LG website that says your model (IPS234V) has speakers. I did however see that it's got a headphone-out socket so perhaps that's all it has for audio. Do you see any evidence of speakers such as two grilles on the back? Is there a volume option in the monitor's settings menu?