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Old July 7th, 2009 (5:50 AM).
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Join Date: Feb 2009
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Some new stuff about GBA music hacking!

A list of commands/arguments from Atrius:
The audio track data is a list of commands and arguments for playing music/sound effects.

Anything below 0x80 is an argument, and anything 0x80 and above is a command.

0x80 - 0xB0 = Rest commands of varying lengths, to wait a certain amount of time before playing each note.

0xB1 = Stop command, to signal the end of the audio track. From what I've noticed even if a song loops it has one of these after the loop command.

0xB2 = Jump command, to jump the playback pointer to another location in memory. Often used to loop songs by jumping back to the beginning of it.

0xB3 = Jump command where playback can jump back to after it again later. Used when you want to repeat part of a song before continuing with the rest of it.

0xB4 = Return to last 0xB3 command.

0xB5-0xBA = Unidentified

0xBB = Set Tempo

0xBC = Set Pitch offset

0xBD = Set instrument

0xBE = Set Volume

0xBF = Set Panning

0xC0 - 0xCE = Unidentified

0xCF - 0xFF = Play notes of varying lengths.

So to break apart the track information I gave you:

BE FF BC 00 BB 01 BD 00-CF 3C 7F B0 B1

BE FF = A volume command with an argument of 0xFF (Volume is an odd exception to the rule that arguments must be less than 0x80)

BC 00 = Set the pitch offset to zero

BB 01 = Very low tempo so that the note is drawn out long enough for voice samples to play their full length.

BD 00 = Set the instrument to the first entry in the instrument map.

CF 3C 7F = Play a note with a pitch of 0x3C (Right in the middle of the available range), and a volume of 0x7F (The maximum available)

B0 = Wait long enough for the note to finish playing

B1 = End the audio track
And some arguments for those commands:

Pitch is very easy to understand. As Atrius noted, it is follows the byte 0xBC (which is usually the first byte in a given track anyhow). Pitch works like this:

0x00 is the base, equivalent to Middle C.

0x0C is one octave higher; High C.

0xF4 is one octave lower; Low C.

If you haven't noticed, it goes by increments of 0x0C, so for higher increments, they would be 0x18, 0x24, ect, while for lower increments they would be 0xE8, 0xDC, ect.

Tracks can be transposed by any value, however, so if you wanted it to be in a different key, you could change these values. If done, make sure to add/subtract 0x0C when raising/lowering octaves.

Tempo is simply the tempo transfered into Hex. So, if your original tempo was 80, you would put in 0x50. It follows a 0xBB byte.

The instruments correspond with the instruments of the voicegroup. Although the instruments can be anything you want, the typical way is to have the instruments correspond directly. It follows a 0xBD The direct map:

0x00 Acoustic Grand (often used as a drum kit)
0x01 Bright Acoustic
0x02 Electric Grand
0x03 Honky-Tonk
0x04 Electric Piano 1
0x05 Electric Piano 2
0x06 Harpsichord
0x07 Clav

0x08 Celesta
0x09 Glockenspiel
0x0A Music Box
0x0B Vibraphone
0x0C Marimba
0x0D Xylophone
0x0E Tubular Bells
0x0F Dulcimer

0x10 Drawbar Organ
0x11 Percussive Organ
0x12 Rock Organ
0x13 Church Organ
0x14 Reed Organ
0x15 Accoridan
0x16 Harmonica
0x17 Tango Accordian

0x18 Acoustic Guitar(nylon)
0x19 Acoustic Guitar(steel)
0x1A Electric Guitar(jazz)
0x1B Electric Guitar(clean)
0x1C Electric Guitar(muted)
0x1D Overdriven Guitar
0x1E Distortion Guitar
0x1F Guitar Harmonics

0x20 Acoustic Bass
0x21 Electric Bass(finger)
0x22 Electric Bass(pick)
0x23 Fretless Bass
0x24 Slap Bass 1
0x25 Slap Bass 2
0x26 Synth Bass 1
0x27 Synth Bass 2

0x28 Violin
0x29 Viola
0x2A Cello
0x2B Contrabass
0x2C Tremolo Strings
0x2D Pizzicato Strings
0x2E Orchestral Strings
0x2F Timpani

0x30 String Ensemble 1
0x31 String Ensemble 2
0x32 SynthStrings 1
0x33 SynthStrings 2
0x34 Choir Aahs
0x35 Voice Oohs
0x36 Synth Voice
0x37 Orchestra Hit

0x38 Trumpet
0x39 Trombone
0x3A Tuba
0x3B Muted Trumpet
0x3C French Horn
0x3D Brass Section
0x3E SynthBrass 1
0x3F SynthBrass 2

0x40 Soprano Sax
0x41 Alto Sax
0x42 Tenor Sax
0x43 Baritone Sax
0x44 Oboe
0x45 English Horn
0x46 Bassoon
0x47 Clarinet

0x48 Piccolo
0x49 Flute
0x4A Recorder
0x4B Pan Flute
0x4C Blown Bottle
0x4D Shakuhachi
0x4E Whistle
0x4F Ocarina

0x50 Lead 1 (square)
0x51 Lead 2 (sawtooth)
0x52 Lead 3 (calliope)
0x53 Lead 4 (chiff)
0x54 Lead 5 (charang)
0x55 Lead 6 (voice)
0x56 Lead 7 (fifths)
0x57 Lead 8 (bass+lead)

0x58 Pad 1 (new age)
0x59 Pad 2 (warm)
0x5A Pad 3 (polysynth)
0x5B Pad 4 (choir)
0x5C Pad 5 (bowed)
0x5D Pad 6 (metallic)
0x5E Pad 7 (halo)
0x5F Pad 8 (sweep)

0x60 FX 1 (rain)
0x61 FX 2 (soundtrack)
0x62 FX 3 (crystal)
0x63 FX 4 (atmosphere)
0x64 FX 5 (brightness)
0x65 FX 6 (goblins)
0x66 FX 7 (echoes)
0x67 FX 8 (sci-fi)

0x68 Sitar
0x69 Banjo
0x6A Shamisen
0x6B Koto
0x6C Kalimba
0x6D Bagpipe
0x6E Fiddle
0x6F Shanai

0x70 Tinkle Bell
0x71 Agogo
0x72 Steel Drums
0x73 Woodblock
0x74 Taiko Drum
0x75 Melodic Tom
0x76 Synth Drum
0x77 Reverse Cymbal

0x78 Guitar Fret Noise
0x79 Breath Noise
0x7A Seashore
0x7B Bird Tweet
0x7C Telephone Ring
0x7D Helicopter
0x7E Applause
0x7F Gunshot (also commonly used as a drum kit)

Volume is a simple concept as well, but unlike most of the song arguments, it must be a value less than 0x80, or else (for some whacked out reason) it acts as a rest o.o

It follows 0xBE.

I need to investigate panning a little more, but I do believe that 0x00 is the neutral value. Either that, or it's 0x0C. I believe that pan's range is from 0x00 to 0x0F.
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