Writing Music

To write in-game music, just write "jukebox commands" in a regular Book & Quill item, then left-click a jukebox with the book to run those commands.

Notes

C#0 - C# in zeroth octave (there are two octaves in Minecraft, 0 and 1)
Ab1 - A flat in first octave

Pauses, beat duration

:-4 - go back 4 beats
d3 - change delay in ticks between beats
-- - 2 beat pause

Chords

C0E0G0 or G0E0C0 or E0G0C0 (C Major Triad on zeroth octave)
C0E0G0Bb, D0F#1A1 (More examples)

Instruments

i0 - change instrument to Harp/Piano

There are 16 instruments total:

i0 - Harp/Piano, the default instrument unless otherwise specified.
i1 - Bass drum.
i2 - Snare drum.
i3 - Sticks.
i4 - Bass.
i5 - Flute
i6 - Bell.
i7 - Guitar.
i8 - Chime.
i9 - Xylophone.
i10 - Iron Xylophone.
i11 - Cow Bell.
i12 - Didgeridoo.
i13 - Bit.
i14 - Banjo.
i15 - Pling.

Volume

v700 - higher volume
v300 - lower volume
v500 - normal volume

Example

i0
d2
E0 - E0 - E0 ---
E0 - E0 - E0 ---
E0 - G1 - C0 - D0 -
E0 -------
F0 - F0 - F0 - F0 -
F0 - E0 - E0 - E0 E0
E0 - D0 - D0 - E0 -
D0 --- G1 ---

:0
C0 -------
C0 -------
C0 -------
C0 --- C0 ---
G1 -------
C0 -------
G1 -------
G1 --- G0 ---