The bug

Some noteblock sounds are not in tune. Note that:

1. Unless otherwise specified, these measurements were conducted using the fundamental pitches (pitch argument of 1) of these sounds played 20 times a second; i.e. The command used is /playsound <sound> master @a ~ ~ ~ 1 1 1 run in a repeating command block. These measurements can be verified with a tuning program or electronic equal tempered tuner.
2. The sounds block.note_block.basedrum , block.note_block.hat , and block.note_block.snare are excluded from this list, as they are not instruments of definite pitch.

These sounds include:

• The block.note_block.bass sound is sharp by about 15 cents
• The block.note_block.cow_bell sound is sharp by about 22 cents (not out of tune when played at its lowest possible pitch of 0.5)
• The block.note_block.didgeridoo sound is sharp by about 6 cents (sharp by 22 cents when played at its highest possible pitch of 2)
• The block.note_block.guitar sound is sharp by about 19 cents

How to reproduce

1. Open a new creative world
2. Run the command /playsound minecraft:block.note_block.harp master @a ~ ~ ~ 1 1 1 in a repeating command block. You should notice that the note is an F# and is relatively in tune.
3. Run the command /playsound minecraft:block.note_block.guitar master @a ~ ~ ~ 1 1 1 in a repeating command block. You should notice that the note is about 19 cents sharp.