A Musical Modulation

When I went into a room, my priest was playing the guitar while singing. (I think that guitar is commonly the background for the singing, like piano is often an accompaniment for violin or singing. That's why I feel that writing "My priest was singing while playing the guitar" is more appropriate.)
It was the first time to me to listen to I heard (or "I listened to") the song.

After that he had finished the play playing (Or "After he had finished the song". "the play" refers to theatre like Noh), I told him that the note  melody seemed to be too high to sing ("too high for his voice" may be clearer), and I tried to sing that song as I felt it would be easier for me to sing.

As After listening to my voice (You could also use "As he listened to my voice", but that would mean he began writing while you were singing), he began to write a modulated version of it (I'm not clear of what "it" refers to. Is it your voice or the song ?) on the lyric sheet.

A ("The" seems more appropriate) skill of musical modulation seems to be wonderful. To me, his doing a musical modulation is no less mysterious than his doing the consecration is. (We can't say "his/hers doing sth", but "him/her doing sth" is possible.)