Midi and certain other Audio playback not working in Windows XP

I am running Windows XP with the latest service pack, and Dragon
NaturallySpeaking Professional 7.0. For some reason, I no longer get any
sound when I ask Dragon to read back a sentence using voice synthesis, or to
play back something that I have just dictated. MIDI sound output no longer
works either, although other sound output does work. I have checked the
sound and audio devices control panel, and can find nothing wrong, except
that Windows XP refuses to associate sounds with sound scheme events. I have
tried reapplying latest service pack, but to no avail. I have tried adding
and removing my sound card. I have tried downloading the latest signed
drivers for my sound card from Microsoft and from Creative Labs. None of
this worked. I just upgraded my processor to an Athlon XP from an Athlon,
but it is quite possible that this problem occurred before my processor
upgrade, and I just didn't notice. I also had a minor problem with a
corrupted file system about a week ago, although I was able to repair the
file system. Does anyone know what could be wrong, and how I might go about
fixing this problem?

I should also point out that if I run dxdiag, I am able to play audio
through any of the MIDI devices on the system.

Thanks,
Michael