Read text one character at a time.

At the minute, I have a very basic (rubbish sounding) synth that plays notes from within the Cmin scale. Most common letters get roots, 4ths and 5ths, whereas the less common get 3rds, 6ths and 7ths.
It responds to live input, but I want it to read from pre-inputted text, and play back at the rate a metro object describes.

All was going well up until this point. Hit a brick wall.
Could you throw some help my way please? I'm stumped.

I have seen this* thread, but I'm doing something a bit different, and I'm not yet experienced enough to take advice from this and manipulate it to my means.

*https://cycling74.com/forums/reading-1-text-byte-at-a-time-from-a-file

Thanks!