Harps come in various sizes. Orchestral ones are very big and no doubt inspired the comic song I took my harp to the party and nobody asked me to play. (An old song, which gives an example of how the meanings of words alter even within living memory.) There are smaller ones, which are used in folk music. Often these are played in groups.
A harpsichord is a keyboard instrument on which the strings are plucked by a plectrum. A hurdy-gurdy is a mechanical instrument otherwise known as a barrel-organ.
A harmonium is a sort of organ with pedals to pump the air. It is not to be confused with a harmonica, which will make an appearance for letter M, or a glass harmonica mentioned in the post for G.
A horn may be a hunting horn made from an animal’s horn, a cor anglais or French horn.
Humdrum is neither humming (singing with the lips closed) nor a drum. It means ordinary.
Harps are mentioned in the psalms including Psalm 149:3