Diagnosis
Dust and fabric grilles are a classic combo. You want to lift the dust out without pushing it deeper into the speaker drivers behind the fabric.
How to Fix It
The best tool for this is a soft-bristled detailing brush, like the kind used for cleaning car interiors or keyboards. Gently brush the grille in one direction (top to bottom works well) to dislodge the dust. Follow immediately with the suction from a vacuum cleaner hose (no brush attachment) held about an inch away to pull the dust away. Do NOT press the vacuum directly onto the fabric, as the suction can be too strong and potentially damage the speaker cones behind it.
If That Didn't Work
For sticky spots or deeper grit, you can very lightly dampen a microfiber cloth with water—just barely damp, not wet—and gently wipe the grille. Immediately dry it with a second clean, dry microfiber cloth. Never spray any liquid directly onto the grille.
Heads Up
The grille is not meant to be removed on this model. Don't try to pry it off, as you'll likely break the plastic clips that hold it on. Always work with the soundbar powered off and unplugged.