I used to use Spotlight to directly start iTunes at a track I wanted to listen to by searching for it. However since upgrading to Lion I've not been able to do this with .m4a music files (mp3s still work). They never show up in spotlight even if I type their name exactly. Has something broken in Spotlight in Lion?
Could it be you installed the 10.7.2 update before Spotlight started working for you again?
The m4a/AAC Spotlight importer was broken in early versions of Mac OS X Lion. It just would not pick up any song metadata. Since 10.7.2 it seems to work again.
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?)