You used Finale, didn't you? Finale with GPO? And somehow recorded directly from Finale? Well, depending on what version you use, there's a place where you can turn off that horrible sewer reverb that's apparently being used in your mp3 file. In 2010, it's in the window where you choose which isntrument bank/set to use. You know, where you choose general midi, or you choose Garritan Personal Orchestra. At the bottom of that window, there should be an option for "ambience," and by default Garritan's sewer reverb is turned on.
If you turn it off, everything will sound 100% better.
If you have no idea what I'm talking about, I can get you some screenshots in a few hours when I'm back on my Mac.
P.S. Sorry it took me so long to get around to this. I totally saw this thread earlier, but for whatever reason I didn't process the issue with the reverb until I randomly came back to this thread just now...