Looking at your comments on your question, you have a 3.5mm cable plugged into the monitor, so lets look at the PC. Now, depending on what your PC's sound card configuration looks like, there will either be (at the rear) a single 3.5mm stereo output with a speaker symbol on it, multiple 3.5mm outputs with different colors, or no outputs at the rear and a headphone and microphone jack at the front of the PC.
You need to plug an audio cable (3.5mm male to male) into the audio input at the rear of your monitor and also to the audio output of your PC.
