For some reason, on my computer, the voice acting sounds like it's being shouted from the end of a long tunnel. It's muffled and echo-y (or something)

I had this game installed on this very computer about a year ago, and I didn't have this sound problem.

I just went to BioWare's site and downloaded the newest patch for BG2: TOB, so I'm out of ideas. I really don't want to play the game with these muffled voices, so any help would be appreciated ^^

That happened to me, but turned out to be the speakers. Are the background noises and music muffled too? If so, I don't know, but if they are normal, that's what mine did. So just reinstall (or whatever) your speakers/sound system.

Try turning off EAX effects in the audio options.

It seems to just be the voices.

Turning off the EAX effects seemed to help it somewhat, thanks Arifirh.

It still doesn't seem to be the way it should be, however. :(

I took the liberty to check the BG2 Tech Support forum at the BioWare forums, I found a topic that seemed to fit with your problem and got solved:

echo effects (http://forums.bioware.com/viewtopic.html?topic=449619&forum=10)

Wow, thanks for your troubles!

That solved my problem perfectly.

Now I can listen to an un-echo'd Irenicus :) !