DIAGNOSIS The speakers in these ultra-slim monitors are basically tiny, low-power drivers. They're designed for basic system sounds in a pinch, not for media or gaming audio. The "tinny" sound is their physical limitation.
FIX The real fix is to bypass them. Plug external speakers or headphones directly into your source device (laptop, phone, Steam Deck). If you must use the monitor's audio, go into your computer's sound settings and ensure the output device is set to the "MNN" display audio, and crank the volume both in your OS and on the monitor's OSD menu (using the joystick/button).
IF THAT DIDN'T WORK Check if you're using an HDMI connection. Sometimes, Windows fails to install the correct audio driver for the monitor's HDMI audio chip. Go to Device Manager > Sound, video and game controllers, right-click on the monitor's audio device, and select "Update driver."
HEADS UP Don't bother with software equalizers to "fix" the sound—you'll just distort it more. The speaker enclosures are too small for any meaningful bass. For anything besides occasional beeps, use external audio.