Velcro hasn’t the holding power for the speaker grills and the adhesive on velcro is not strong enough either.
To bring your A game switch to Dual-Lock. It’s a 3-M product that is similar to Velcro. You may have seen it already. Your toll pass (E-Z Pass?) is attached to your windshield using it, under bright sunlight, in all temperatures, with extreme vibrations for years at a time. I think it might prove to be an answer.