Some time back, I posted a somewhat desperate question about my efforts to "play around" with the Beat Box app in Head First Java. I have now found the answer to my problem, with lots of help, in a very unexpected (to me, at any rate) place; since it applies to lots in addition to my original problem, I'm starting another thread.
I had Java automatic updates enabled on my computer. It turns out that in addition to installing the updates in the Program Files\Java folder, the executable files java.exe, javaw.exe and javaws.exe were also being inserted into my WINDOWS\SYSTEM32 folder. The update log does not mention this. It further seems that in spite of my setting my path correctly (I thought, anyway; it's been working for a while), the command prompt "java ClassName" looks first in the windows directory for an executable. Since the windows\system32 folder does not contain all of the subsidiary files, this is not a good thing. Specifically, the audio file which contains the Java sound bank does not exist in this directory. When I disable (by renaming) the executables in the system32 directory, everything works just beautifully.
So I have disabled automatic update and posted a question about it on one of the Sun forums. I suppose I could put the "java" path at the front of my list of paths, but I don't think that this should be necessary ...
In case anyone else is having really weird problems, maybe this could help.