So that would be from the 3.5mm jack on your motherboard to the 3.5mm jack on the monitor.
You can test each of them to see which is the issue.
For the motherboard output, connect headphones or other speakers and see if the audio is working. If not, look at drivers and audio settings in Windows.
For the monitor, connect another audio source like an mp3 player. If not working, check monitor input settings.