No sound, but eveything looks fine!

Background:

The sound was working perfectly fine up until something unknown happened. The volume wasn't high enough for my taste despite volume being set to highest, so I plugged in an external speaker. Nothing bad happened. After unplugging it, sound stopped altogether (now, sound won't play through either headphones or monitor speakers). I did play around with Realtek's Audio Configuration tool, but this should have nothing to do with the problem, since it's an apparently OS-independent problem. Besides, I reinstalled the driver (which the Audio Configuration tool uninstalls with).

Hardware Info:

Windows XP, SP2
Realtek AC'97 Audio for VIA Audio Controller
EliteGroup P4M800PRO-M motherboard

Q&A:

Is the volume muted? No.
Did you install the proper driver? Yes (from the website of the motherboard manfacturer and also tried Realtek's own driver).
Does Device Manager show any signs of audio malfunctioning? No.
What is the device status in Device Manager? "This device is working properly."
Did you do any physical damage to the computer? No, but if I did, Windows likely wouldn't recognize the on-board audio (it does recognize it).
Is the problem operating system related? Apparently, no, because it doesn't work under Ubuntu either.
Is the audio enabled in the BIOS? Yes.
Is everything connected properly? Yes.
Are you using Realtek's HD driver? No.
Did you Google the problem before asking questions? Yes.

Any solutions?

I like that Q&A section. It really helps to focus on an answer.

If any damage was done to the mother board, it might have happened if you unplugged the speakers while the PC was running. I know that it shouldn't make a difference but somehow I think that's what happened.

I would install a sound card. To get the volume and the sound I want, I bought a cable for 10 dollars that enables me to connect my sound card to my amp and preamp on my home stereo. It rocks. It beats any PC speakers on the market. Go for it.

Just for kicks, look at the event viewer.
start, run, eventvwr.msc

This topic is somewhat moot now because I bought a sound card. My problem must have been a hardware one because this new sound card works perfectly fine.