A lot of my iTunes files are protected and unavailable to use in iMovie. Why is this?

check in Finder the suffix of those audio files ... if it's .m4p, those are protected=DRM=not meant/allowed for 'processing' aka using in videos...,

only legal way is:

* create a playlist of your m4p filesn in iTunes

* burn a CD from this playlist in iTunes (... if your Mac comes with a disk-drive ...)

* import this CD back to iTunes => same title, but now with .m4a = audio, unprotected... could be helpful to add some 'tag' in iTunes to those re-imported music to indicate them as 'processable'...

There is an additional legal way. Get iTunes Match for about $25 a year. You will have access to all your music in the cloud. After synching with iTunes Match, you can delete your local copy-protected copies and then redownload the DRM-free copies from iTunes Match.

This worked for me. Thank you.

I just find it frustrating that music I have purchased through iTunes is unavailable for all Apple apps. I have just wasted two hours of valuable time researching and implementing a way to add a "protected and unavailable" track that I own to a family movie.

Most people buy Apple because "it just works", but that is no longer strictly true.