DIAGNOSIS The built-in speakers on most budget monitors, including this one, are essentially emergency audio. They're small, low-power drivers that can't produce good bass or volume, so this is more of a design limitation than a defect.
FIX Your best fix is to bypass them. Get a 3.5mm audio cable. 1) Plug one end into the monitor's 'Audio Out' port. 2) Plug the other end into the 'Line In' or 'Aux' port on a decent set of external speakers or your PC's speaker system. Now, audio from the HDMI input will pass through the monitor to your good speakers. Alternatively, just plug your headphones or speakers directly into your computer's audio jack, cutting the monitor out of the audio chain entirely.
IF THAT DIDN'T WORK If you're determined to use the built-ins, double-check your Windows sound settings. Right-click the speaker icon, open 'Sound settings,' make sure the correct output device is selected (it should be the monitor), and crank the volume slider there to 100% as well as the monitor's OSD volume.
HEADS UP The audio quality will always be poor because of the physical speaker size. For gaming or any real media, a $20 pair of external desktop speakers will be a massive, night-and-day improvement.