Music theory has the concept of "octave equivalence", where changing the pitch of a note up or down by an octave does not change its function. This means it's possible to enumerate all the functionally distinct harmonies available in standard Western tuning. There are 12 distinct pitch classes. One of these must be the lowest note in the harmony, so there are 11 remaining pitch classes that could optionally be included or omitted. If all are omitted it's just a single note not a harmony, so there are 2^11-1 = 2047 functionally distinct harmonies.

Most of them are of little musical utility, but you could conceivably use and have opinions about them all. This is not possible with literature. There are far more than 2047 functionally distinct sentences, so there is no need to add more letters to the alphabet.