Happened to me on Windows 7 x64 professional PL RTM now. Initially, it was working fine, now no mp3s are ever registered as recent items.
For example, Windows Media Player will register avi files and show them in the recent items list, but not mp3s.
Initially I thought it was a problem with the winamp taskbar integration plugin, as I was using winamp primarily, but then I realized that the arrow right of the winamp shortcut in the start menu opens nothing. Other file types work fine, just not mp3s.
Interestingly enough, links to opened MP3 files are present in the C:\Users\[username]\AppData\Roaming\Microsoft\Windows\Recent folder. They just aren't attached to the programs using them for some reason.
PS. If I activate the old-style recent items menu, the mp3s show up there, but they still don't show up for neither winamp nor windows media player.
My OS is Windows 7 x64 Professional PL.
Interestingly enough, Windows Media Player has avi files in its recent items list despite that these files aren't stored in the recent items folder.