In foobar2000 0.9.1 under WinXP Pro SP2, if you drag a directory containing .M4A files from Windows Explorer onto a playlist, no items are added. The same action will successfully add MP3 files to the playlist. M4A files are associated with FB2K and if dragged individually onto a playlist are correctly added and played.
(Also: a quick O/T query: How does one remove custom fields from the "Add New" tagging function in the Properties page for a track?)