This is what the release notes say
When MP3 files are added (either manually or automatically) to either the Windows Media Player or the Windows Media Center library, or if the file metadata is edited, several seconds of content may be permanently removed from the start of the file. This issue occurs when files contain thumbnails or other metadata of significant size before importing or editing them.
To avoid this, ensure that all MP3 files that may be accessed by a computer running Windows 7 (including those on removable media or network shares) are set to read-only. To do this, in Windows Explorer, find the files, right-click them, click the General tab, and then select the Read-only check box. Then back up all of the MP3 files prior to using Windows Media Player or Windows Media Center.
If some of your files have already been affected, you may be able to recover the data by using the Previous Versions feature. To do this, right-click the file name, click Properties, click the Previous Versions tab, and in the File Versions pane select the most recent previous version.