My wifes EX does the same, but only when the bass is turned up too much for the speakers. Turn it down and see if that helps any. If it still sounds scratchy or muffled at lower bass levels, than you've more than likely blown the speakers. They're easy to replace, and even cheap aftermarket ones will be a nice improvement over stock.
'05 Sorento LX 4x4. Purchased 9-14-11 43,800mi. "Karinka"
'08 Spectra EX 5speed. Purchased 3-25-12 43,500mi. "Saki"
'99 Sephia 5speed "Kurumi". Traded in 2006.