When I browse to the directory containing Google Drive, the Win8 XBox music app sees the other directories and even SkyDrive and DropBox, but not my Google Drive.
The directory is here: C:\Users\v-scslac. Interestingly Google Drive only appears in the explorer view, not from the command line.
C:\>dir /w C:\Users\v-scslac
Volume in drive C is OSDisk
Volume Serial Number is E429-D0B7
Directory of C:\Users\v-scslac
[.] [..] .gitconfig [.ssh] [AppData] [Contacts] [Desktop] [Documents]
[Downloads] [Dropbox] [Favorites] [Links] [Music] [Pictures] [Saved Games] [Searches]
[SkyDrive] [Tracing] [Videos]
1 File(s) 53 bytes
18 Dir(s) 394,041,049,088 bytes free
It appears that their File System Driver is not up to the level that SkyDrive and DropBox is. Interestingly, Windows Media Player can see it and play it.