If you’ve downloaded Windows 7 beta 1 build 7000 leaked over the internet has a bug: The problem only happens when you edit metadata on a certain class of MP3 file (a file with a header larger than 16KB). When the new metadata is written, it corrupts the beginning of the file. This can happen either when you edit the metadata from inside WMP or Explorer, or it can happen if you have WMP set to automatically fill in missing metadata using the online service and add the MP3 to your library.
Microsoft's official response is "Microsoft is aware of this issue and it will be addressed in the beta program". The company is currently hard at work readying a QFE (quick fix engineering) patch that will be distributed by Windows Update for those with official and non-official access. A support (KB) article is expected soon.