Hi,
When you find track that MMA can't find, try to find it using MM5+ and if it is found or not, then please Open support Ticket, attach DL link to file so that we can try to find Lyrics source (if possible send us link to correct lyrics from Google search)

That will help us to determine if it is possible for MM to use that source so that we/or someone can create addon for search.

No, this is not use customizable.

Is there a way to change where MMA searches for lyrics? I'm finding that it fails to find lyrics that a cursory Google search finds easily.