A bug in Windows 7 Beta 1 (build 7000) is known to corrupt MP3 files if you use the Windows Media Player to modify or automatically update (the default setting) MP3 file meta data.  Microsoft has released a QFE patch and the bug has been fixed in more recent builds.

If you are running Windows 7 Beta 1 Build 7000 be sure to download the patch:

Windows 7 Beta 1 32-bit Patch Download
Windows 7 Beta 1 64-bit Patch Download
Read More in KB961367