Earlier this week, Microsoft warned that there was a serious MP3 corruption issue in the Windows 7 beta (build 7000), which has been making the rounds on piracy networks. The company promised a fix, and it has now arrived. Regardless of where you got the beta from, I strongly recommend that you download and install this patch: 32-bit and 64-bit. The accompanying KB article 961367 has not yet been posted as of publishing time. Nevertheless, the Microsoft Download Center describes the fix as follows:
Install this update to resolve live and recorded TV issues in Windows Media Center, recorded TV playback issues in Windows Media Player, and MP3 file corruption issues in Windows. After you install this item, you may have to restart your computer. This update is provided to you and licensed under the Windows 7 Pre-Release License Terms.