Why is my iTunes suddenly playing quiet after import of music?

Hi all,

Wondering if someone can help? I've read a lot of posts regarding this but can't find a solution.

My iTunes has always played fine but after importing some music off a hard drive today it has played quiet (not so loud on full volume and goes to silent at half volume).

I have checked and unchecked 'Sound Check' in iTunes Preferences...no difference.

I have reset the NVRAM.

I have checked all sound levels on my computer (sound fine playing in all other ways on my MBP).

I have run Diagnostics (just in case) - no problems.

If anyone can shed any light I'd be really appreciative.

Thanks!

Bonnie