Being in the plastics industry, this is usually caused by not enough UV inhibitor in the formulation. Accelerated environmental aging studies usually bring this to light fairly quickly. The other thing that happens with most organic things (plastics included) is oxidation. If the plastic used here was compounded without it's UV and anti-oxidation package, or not enough of it, that could explain the results seen. Unfortunately this process (compounding) happens well before the plastic pellets get to the injection molder Axiom uses for their speaker housings.
Krylon makes a line of paint for plastics, but this is probably where the warranty would come in to play first.
Scott
