Toyota fixed the dash cracking problem, so I don't think it's been a problem except for early 4th gens. A dash mat could actually make it worse if it makes the dash get hotter, somehow, by absorbing or trapping more heat. It's unlikely, but who knows? A dark color will absorb light and get hot, and heat could be trapped between the mat and the dash rather than getting airflow that it would get if the dash was uncovered. A light color could reflect more light onto the windshield and cause more glare in some conditions.
High frequency sound is very easily blocked, and even a medium layer of fabric could attenuate it some. Dash mat certainly looks thick enough to muffle high frequency sounds. The best thing to do would be to cut out holes in the dash mat where the speakers go, and optionally sew in panels of acoustically transparent fabric like what's used to cover speakers and sound absorbing panels. Maybe see if they can do it for you for a clean look.