It all depends, no simple answer.
First, your Spotlight index might need re-generation. From a Terminal window, do the following: sudo mdutil -E /
Once that's done, it still depends on where/how the information is stored. For example, iTunes "Description" field does not show up in Spotlight searches, but if the file has the Description in the file's comment metadata field, then will see.
Pictures in Apple's Photos might not show as the app stores metadata in their own database, and only gets tacked onto the actual photo file when exported and you click the "retain" checkboxes.
Things like Office documents, Office calls a helper program they have to provide keywords to the Spotlight index when you save a document.
You might want to see what information is saved on one of your "missing" audio files via the "Get Info" menu option in Finder: should give you an idea why/why not showing up.