Microsoft has issued the first patch for the beta version of Windows 7, it fixes a file corruption bug that shaved several seconds from MP3 files. Additionally, the patch also fixes some playback and recording issues. You can get the update over here.
An MP3 file support issue that is fixed in this update
Every time that metadata is edited in an MP3 file that already contains lots of metadata in the file header, some audio at the beginning of the track may be lost permanently. Up to several seconds of audio may be lost.
Large headers are common in music files that are purchased from commercial services because the files usually have large album art. Use of tools to add large album art to existing MP3 files may also cause this audio loss. Specifically, any information that causes the header size to exceed 16 kilobytes will trigger the loss. Each edit will increase the total loss.
Important: Windows Media Player has a feature that adds metadata that is missing. This may include large album art. By default, this feature is turned on. Therefore, without action on your part, all MP3 files that have large headers in your Windows Media Player and Windows Media Center libraries are likely to lose some audio. Install the update to resolve this issue before you introduce MP3 files to the computer.