Spotlight can't find m4a files
Try opening up the Spotlight Preferences and re-ordering the position "Music" by dragging it higher up in the list. In Lion when you search via Spotlight it prioritizes the most relevant items according to the order you see in the Spotlight Preference Pane. 

Another thing that will save you some frustration is to do a Spotlight search (any file will do) and select "Show All in Finder", from there select "Show View Options" and change Arrange by to "None" and Sort by to "Name".

If those two things don't work then I would recommend rebuilding your index. (Do I need to re-index my Volume?)