Learning to play the instrument by ear is a completely different thing from learning to play a song by ear. My bachelor's is in classical guitar performance, b

Learning to play the instrument by ear is a completely different thing from learning to play a song by ear. My bachelor's is in classical guitar performance, but I've been playing all sorts of instruments my whole life. If I hear something "new" and play it by ear it's because it sounds similar to another thing that I've learned to play before. Learn a whole bunch of songs, and eventually you'll hear something "new" and say, "wait a minute that's not new to me, that's only a little different from the last song I learned." I recommend that you learn using every resource available to you, build a vocabulary of music you can draw on while you're at the harmonium, and then go about discovering music by ear. Forcing yourself to learn the skill of harmonium playing only by trial and error is an unnecessary constraint, and it will take you much longer to play fluently by ear.