If you want a good ear training challenge, put on a recording of some choral music and listen to the alto part throughout.
Your ear will naturally gravitate towards the higher soprano part, but try to ignore that. Cue in on the slightly lower notes also sung by female voices; the alto part.
It may be difficult at first, but with enough practice you’ll begin hearing those inner voices with more clarity. They’ll start popping out at you and you’ll be amazed at how much you may have been missing all these years. And as an added bonus, you’ll start hearing those same inner voices when you play piano. Stick with this for a while and your ear will get better. 🙂
HERE'S what I'm thinking while you practice piano.