With this resource pack you can play the default harp with 2 more lower octaves and 2 higher octaves for 1.15.2 Basically I replaced the "pling" instrument (noteblock over glowstone) with the harp but 2 octaves higher and the "bit" instrument (emerald block over noteblock) with the harp but 2 octaves lower, hf.
NoteblockPack.zip