Medium (disc) titles are not always present like a barcode or a label might be, thus the default text should not say '[Unknown]' but should say something like "Add title".

"Add disc title" would be more descriptive and less confusing, but it's not always vinyl or optical media so "disc" may not always apply.

"Add medium title" would work, but I would like to see "medium" used as infrequently as possible, I don't think it's a very user-friendly word to use all the time (especially given the international user base).