DIAGNOSIS: The grille is a magnetically attached woven polyester fabric. You can't scrub it like a hard surface, and liquids are a big risk.
FIX: Remove it first. Gently pull from the bottom-center of the Arc; the magnets will release. Use a soft, dry brush attachment on your vacuum cleaner on the lowest suction setting. Gently run it over the fabric to pull dust out. For smudges or spots, a barely damp (not wet) microfiber cloth can be used with a light dabbing motion. Let it air dry completely before reattaching.
IF THAT DIDN'T WORK: For tougher, set-in stains, Sonos officially says you can use a very mild, non-abrasive cleaner diluted in water. Apply only with a damp cloth, never spray directly onto the grille, and blot—don't rub—the area.
HEADS UP: Never put the grille in water, use a washing machine, or apply heat (like a hair dryer). The fabric can shrink or the backing can warp, and it won't fit back on properly. Also, don't try to clean it while it's attached—you'll just push dust into the speaker slots.