wma music files sound bad

I have been experiencing problems with my music (wma) files stored on my hard
drive. After a while, certain files playback with distortion in the higher
frequencies and an overall loss of dynamic content. They sound like a low
bit-rate, compressed mp3 (on both windows media player and dell musicmatch
jukebox), even-though most of my music was recorded as ether a wma or wma
lossless file. Could this be the result of disk defrag? Can anyone tell me
how to maintain the integrity of the original file information? Here’s some
of my system info:

Windows Media Player 11.0.5
OS Name Microsoft Windows XP Home Edition
Version 5.1.2600 Service Pack 2 Build 2600
System Manufacturer Dell Computer Corporation
System Model Dimension 4600i
System Type X86-based PC
Processor x86 Family 15 Model 3 Stepping 4 GenuineIntel ~2992 Mhz
Processor x86 Family 15 Model 3 Stepping 4 GenuineIntel ~2992 Mhz
Boot Device \Device\HarddiskVolume2
Total Physical Memory 1,024.00 MB
Available Physical Memory 524.52 MB
Total Virtual Memory 2.00 GB
Available Virtual Memory 1.96 GB
Page File Space 2.40 GB