Your or someone elses ears... There isn't another reliable choice apart from melodyne, which IS costly and is not exactly used for such purposes (it's used to correct notes which are off tune, etc), but is great at analyzing what's going on even in polyphonic passages!
The mp3 to midi choices are all rather poor efforts that never work too well.
And then, of course you'd have to transcribe the midi into a score, which is another thing, but still.
to be honest through here I met someone who requested a couple of video game tracks transcribed for piano, in a score format. I did do it but I was paid for that...
I have a tendency to figure it out measure by measure...unless the melody line comes across easliy....then I might write it down and figure out the progressions/arrangments of notes used Sometimes it goes faster than others. All depends on the piece. the instrument used and what my ear is able to figure out. Can be fun, Can be time consuming.
Depending how fast or involved a passage is, I open the file in Goldwave and slow it down while keeping the same pitch. This helps me not have to listen to a passage a hundred times. I typically get the harmonic structure first, chart the melody and chord progressions, then the bass. Obviously inner voices are the most difficult. But transcribing is a great discipline to learn, and develops your ear to listen in layers.
Hugh Poland 1924 Knabe 6'4" Grand
I sounds like a hymn. I never grew up with hymns and I also never grew up where students sang 'good morning dear teacher'...heck hardly anyone recites the 'Pledge of Allegiance' anymore.
Here's the songs translation.
It's time for us to leave today 'cuz it's the end of school. We've learned our math and english too, we know the golden rule. We promise to do all our chores and brush our teeth 'fore bed. Then angels will watch over us as we lay down our head.