Womble DVD does a good job of converting 2.0 into 5.1.
You can also go through the probably-tedious process of cutting out everything that isn't dialog (on the center channel).
http://www.ohmss-007.com/neversaymcclor ... ive01.html
His approach starts with 2.0, but it'd work for editing the center channel. I think it'd be better to use the center, than 2.0, because there shouldn't be as much volume in the music, compared to the voice, as there would be in 2.0.
You might
have to add sound effects &/or music back in, to hide the editing, though, I don't know.
Dialog is almost always nailed onto the center channel in the movies. (Games might be different. But you can use Graphedit and PowerDVD or Windvd to make 5.1.
http://forum.doom9.org/showthread.php?t=83384
(Thanks to Doctor M for giving me that link, after I had lost track of it).
The reason for going to that trouble - it gives you the option of making adjustments. You can adjust it so that it grabs more of the sides and puts it in the center. That way you can center the dialog that isn't quite centered.
Might also be able to use that Graphedit technique to bring part of sound effects, from your side channels, and put them into the center. (After you've edited everything but voice from the center).
That'd let you cut out dialog, without replacing sound effects. You could try it with music, too, but I don't know if the music would sound right.
If the sound effects are mixed in with music, then you can't separate them. But you could try to steal them from another episode/movie, if you can't find them in a sound effects collection.
ADM likes to do a hard-cut of the dialog, on the center channel, but do a long crossfade on the other channels, to blend the music.
Some movies have wierd sound. Wrath of Khan, for example, is a goofy exception to the dialog-in-the-center rule. It sounds like they recorded everything in stereo and used the stereo for the side channels, then made a mono, of the stereo, for the center, and then added extra sound effects for the rear. You can still process it to get a center channel that's mostly voice, but you'd have to throw away the side channels & remake them from the soundtrack cd & sound effects.
Keep in mind, I haven't tried any of this, except for making 5.1 with Graphedit.
You'll pretty much need Vegas, or something similar, to do any of the 5.1 editing right.