Hi Jeff:
I don't have any answers -- but if this were happening to me, I would:
Take one of my files (BTW, what kind of files? WAV, MP3?) to a friend's computer and play it through HIS sound card. Does it sound OK with Windows Media Player? If it does, see if your friend will let you load a free-trial version of WAVEPAD on his computer, and see if you hear/see your "fading" symptoms there. (WAVEPAD can easily be removed from the INSTALL/REMOVE PROGRAMS window in START -> SETTINGS -> CONTROL PANEL, if your friend doesn't want to keep the trial version around and play with it himself. I've had no trouble with the WAVEPAD uninstaller at all.)
If you DON'T hear the fading portions in WMP, but DO hear them in WAVEPAD, then you might try another free editor program called AUDACITY -- not as many bells and whistles as WAVEPAD, but it would be a good test to see if the same problem occurs there.
http://audacity.sourceforge.net/
If the problem appears with ALL the programs on your friends computer, I think your files are (permanently) recorded that way (probably from the initial stream through your sound card). You might be able to fix up the fading with some AGC (Automatic Gain Control), but if it's music, that's probably going to wreck the dynamic range.
If all the evidence points to your sound card, you might be able to confirm it by having your friend make a recording on his computer at the same time you record the same program on your computer, and then compare the two files. If your friend doesn't have a good easy-to-use sound-card-recorder, I recommend FREE HI-Q RECORDER. It installs (and uninstalls) easily, and is really easy to use. It's a crippled version of the shareware EASY HI-Q RECORDER. The download is here: (Scroll down for the FREE version.)
http://www.roemersoftware.com/
Good luck.
Jerry