I would doubt most bards could afford books until after the printing press (~1440). During the middle ages (500-1500) is after the fall of the western roman empire (470) where papyrus stopped coming in for about 600-700 years (1100s) before cheap paper from Spain. I think court musicians were a bit different in “class” someone traveling is unlikely to be bringing a lot of written stuff with them all the time unless they were a weirdo. Once cheap paper and moreso the printing press to allow cheap copying spread then so did literacy. So its hard to definitely say whether the average bard would be literate. I think at the start of the middle ages, no, but by the end of the middle ages, yes.

More likely would have been taught orally. A traveling minstrel type bard is unlikely to have written music. Learn the song from your master/other performers then adapt as you wish.