yes the cones do dry out and crack after awhile. if you are not into taking them apart and finding suitable replacements(sometimes better ones can be found) and soldering them back in and reassembling them its better to just replace all together.
however if you do take them apart and dont feel like chasing them down on amazon or wherever, often radio shack has pretty decient replacements. ALSO you can also see their line of computer speakers there and weigh out your options...