I don't have much confidence in the general idea of singers reading music. Most people can develop a good sense of what the music sounds like, but to get it absolutely right, you need to hear it. The best way to do this, apart from music school, is to become a Lutheran.

If you play an instrument, sheet music is good at telling you where to put your fingers, but the skill doesn't transfer 100% to the voice. For one thing, two songs can look identical, but if they are in different keys, the intervals are different.

I have sung with music majors, and most of them wobble on first sight, too. When I sang in a large choir that did concerts, the leaders made recordings of the parts, and we learned off of them.

(Ah, Christmas and Eastertime, when you pull up at a stoplight and see fellow drivers singing their parts on the way to work.)

So the monk Walter has my sympathy. His teacher is demanding the impossible. Walter won't get better at reading by studying alone in his cell. He needs to follow the notes while hearing others sing.