Diagnosis
The grille is a removable acoustic fabric. Do not spray cleaners directly onto it. Liquid can seep through and damage the drivers behind it.
How to Fix It
Power off and unplug the soundbar. Gently pull the grille straight off from the bottom corners. Use a soft-bristled brush, like a clean paintbrush or a dry microfiber cloth, to lightly brush away dust. For stains, slightly dampen a cloth with water, wring it out completely, and dab (do not rub) the affected area. Let it air dry fully before reattaching.
If That Didn't Work
For ground-in dirt, you can use a vacuum cleaner with a soft brush attachment on the lowest suction setting. Hold the grille firmly and run the brush lightly over the surface.
Heads Up
Never submerge the grille in water. Do not use solvents, alcohol, or abrasive cleaners. The fabric is not designed to be washed. Reattach the grille by aligning the plastic clips and pressing firmly until it snaps into place evenly.