Start by clicking on the little wrench icon on the ingame runescape menu bar, is the sound turned on? if not run it on. Is the music turned on? if not run it on. Are sound effects enabled? If they are DISABLE THEM they are VERY annoying.
if all of those things are switched on try downloading the latest version of java from either
http://sun.com or
http://java.com .
if that is still not working try switching your version of java. You can do this at the bottom of the detail select page.
If it is still not working then you should either send in a support email or consult the runescape knowledge base at
http://runescape.com/kbase