For Super Mario 64 hacking, there are several ways to create a custom music sequence.
If you are working with the decompilation project, you may write down a music sequence in assembly. The decompilation project provides macros that helps writing music files. See includes/seq_marcos.inc for available commands.
The following code snippet is a short version of the popular Tetris theme. Put this file into the sequences folder (e.g. sound/sequences/us), give it a unique hexadecimal ID in the filename or replace an existing track. Use .s (assembly) for the file extension. In sound/sequences.json you need to choose a sound bank to use. Since this snippet is using instrument 0 only, it works well with most sound banks by default.
.include "seq_macros.inc" .section .rodata .align 0 # instruments to use .eqv INSTR_MELODY, 0 .eqv INSTR_BASS, 0 # note values .eqv WHOLE, 192 .eqv QUARTER, 48 .eqv EIGHTH, 24 .eqv QUARTER_DOTTET, 72 .eqv HALF, 96 # note pitches .eqv A4, 48 .eqv G4, 46 .eqv F4, 44 .eqv E4, 43 .eqv D4, 41 .eqv C4, 39 .eqv B3, 38 .eqv A3, 36 .eqv E3, 31 .eqv D3, 29 .eqv B2, 26 .eqv A2, 24 .eqv E2, 19 sequence_start: # this label is required at the beginning of the sequence seq_settempo 125 # in BPM (beats per minute) seq_setvol 127 # 0..255 seq_initchannels 0x03 # bitmask; use channel 0 and 1 seq_startchannel 0, .channel0 # we play melody here seq_startchannel 1, .channel1 # we play bass here seq_delay WHOLE * 8 seq_jump sequence_start # endless loop seq_end .channel0: chan_largenoteson chan_setvol 127 chan_setinstr INSTR_MELODY chan_setlayer 0, .layer_melody1 chan_delay WHOLE * 4 chan_setlayer 0, .layer_melody2 chan_delay WHOLE * 4 chan_end .channel1: chan_largenoteson chan_setvol 127 chan_setinstr INSTR_BASS chan_setlayer 0, .layer_bass1 chan_delay WHOLE * 4 chan_setlayer 0, .layer_bass2 chan_delay WHOLE * 4 chan_end .layer_melody1: layer_note1 E4, QUARTER, 100 layer_note1 B3, EIGHTH, 100 layer_note1 C4, EIGHTH, 100 layer_note1 D4, QUARTER, 100 layer_note1 C4, EIGHTH, 100 layer_note1 B3, EIGHTH, 100 layer_note1 A3, QUARTER_DOTTET, 100 layer_note1 C4, EIGHTH, 100 layer_note1 E4, QUARTER, 100 layer_note1 D4, EIGHTH, 100 layer_note1 C4, EIGHTH, 100 layer_note1 B3, QUARTER_DOTTET, 100 layer_note1 C4, EIGHTH, 100 layer_note1 D4, QUARTER, 100 layer_note1 E4, QUARTER, 100 layer_note1 C4, QUARTER, 100 layer_note1 A3, QUARTER, 100 layer_note1 A3, HALF, 100 layer_end .layer_melody2: layer_delay EIGHTH layer_note1 D4, QUARTER, 100 layer_note1 F4, EIGHTH, 100 layer_note1 A4, QUARTER, 100 layer_note1 G4, EIGHTH, 100 layer_note1 F4, EIGHTH, 100 layer_note1 E4, QUARTER_DOTTET, 100 layer_note1 C4, EIGHTH, 100 layer_note1 E4, QUARTER, 100 layer_note1 D4, EIGHTH, 100 layer_note1 C4, EIGHTH, 100 layer_note1 B3, QUARTER, 100 layer_note1 B3, EIGHTH, 100 layer_note1 C4, EIGHTH, 100 layer_note1 D4, QUARTER, 100 layer_note1 E4, QUARTER, 100 layer_note1 C4, QUARTER, 100 layer_note1 A3, QUARTER, 100 layer_note1 A3, QUARTER, 100 layer_delay QUARTER layer_end .layer_bass1: layer_call .layer_fn_bass_e layer_call .layer_fn_bass_a layer_call .layer_fn_bass_e layer_call .layer_fn_bass_a layer_end .layer_bass2: layer_call .layer_fn_bass_d layer_call .layer_fn_bass_a layer_call .layer_fn_bass_e layer_call .layer_fn_bass_a layer_end .layer_fn_bass_e: layer_loop 4 layer_note1 E2, EIGHTH, 100 layer_note1 B2, EIGHTH, 100 layer_loopend layer_end .layer_fn_bass_a: layer_loop 4 layer_note1 A2, EIGHTH, 100 layer_note1 E3, EIGHTH, 100 layer_loopend layer_end .layer_fn_bass_d: layer_loop 4 layer_note1 D3, EIGHTH, 100 layer_note1 A3, EIGHTH, 100 layer_loopend layer_end