I have a home theater computer running Windows 7 and WMC. It is a fresh install from 5 days ago, fully patched/updated, though I have not installed WMP 12.
All my music is in a second hard drive (D:\Music).
I share D:\Music with my network which consists of two Mac OS X 10.6 laptops.
I purchased a new album on iTunes (BT's These Hopeful Machines for those curious :)).
Only three of the songs from the album (track 2, 5 and 8) appear in Windows Media Center or Windows Media Player. The songs are M4A format. All the songs in the album play just fine if I double click them in Explorer.
The folder (D:\Music) is in my Media Center/Player library for Music.
Why don't the remaining songs show up? I can play them, they just won't get added to the library. I do have other albums that have this problem, but this is the most recent and I imagine the symptom is common between them.