You can change the wheels yourself by using an allen key ( click
here if you don't know what that is) to loosen the bolts on the wheels. Go to any skate shop to get wheels. But try
this anyway.
As for why the wheels will crack...well I don't know about the melting part but some older inlines come with wheels which are not a whole solid piece. As in the wheel has two layers. The outer layer may tend to crack if its too old.
If the wheel is a solid one and it has cracked, then it could be that its really really old and that it had been under a lot of stress. Either that, or it just sucks.