Take a look at the wind blox link. The methodology accounts for the supposed issues you discuss.
I've used a sound meter app and the figures are very close to the approximations in the graph I posted (and to the wind blox measurements as well).
To put it succinctly, there's little risk to your hearing unless you ride 22+ mph every day for 8+ hours a day.
I've tried cat ears and haven't had much luck with them. The problem is one of variable winds. Also, your helmet straps need to be very close to your ears and the location of straps varies considerably from one helmet to the next.