In the Sound tab from the Preferences dialogue box, no instrument can be found.

Sorry, this problem has already been spotted. It is because of the java virtual machine. This problem is totally random. For example, in my case, I have a computer which works with Linux and Win 98. With Win 98, the list doesn't appear and with Linux, it does!! And I'm using the same JRE and have no material problem. This may change from a JRE version to another.