0-6: Percussive chordophones and idiophones
7: Xylophone?
8: Tubular Bells
9: Vibraphone?
10: Music Box
11-12: Idiophones
13: Accordion
14: Acoustic Guitar
15: Acoustic Guitar
16: Overdriven Guitar
17: Overdriven Guitar
18: ???
19: Upright Bass
20: ??? (a bass instrument)
21: Slap Bass
22: ??? (a bass instrument)
23: String Ensemble
24: Viola or Violin (I'm not sure)
25: Cello
26: Pizzicato Strings
27: Harp
28: Timpani
29: Trumpet
30: Tuba or Trombone
31: ??? (Some type of brass instrument)
32: Tuba or Trombone
33: ??? (It's a high pitched aerophone, maybe an Oboe or Piccolo)
34: Clarinet
35: Flute
36: ??? (Aerophone)
37: Orchestra Hit
38: ???
39: Drums
40: Some weird Drums
41: ???
42: Japanese chordophone (don't know the name)
43: Not an instrument
44: Wind Sound Effect (used in the Frontier Brain battle theme and the Battle Arcade)
45: ??? (It sounds terrible and I haven't encountered a song that uses it; may be used as a sound effect)
46: ??? (Electrophone used in Silver's battle theme and another battle theme)
47: Bubble Sound Effect (used in Lugia's battle theme)
48: Square (as in GB quality)
49-51: Not an instrument
52-53: Squares (used in the credits)
54: Not an instrument
55: Whistle
56: ??? (sounds like a synthetic cymbal)
57: a type of Organ
58-59: Not an instrument
60-69: Vary depending on voicegroup (Includes the Grand Piano used in National Park, the Synthesizer used in many battle themes and the Global Terminal, an electronic drum set used in most battle themes, a few choirs used in some battle themes and Union Cave, a few Orchestra Hits that are used in battle themes, Percussive Bells, traditional instruments used in Ecruteak City and the Burned/Bell Towers, a Saxophone used in a couple Battle Frontier facilities)
70-89: Not an instrument
90-127: Some of these are the sound effects used when walking on grass, dirt, a road, etc. Also includes the wave sound effect when standing by the ocean.
Gamefreak did that to throw us off, too bad we're SMART
lol I think the different instrument numbers are because of the software that the composer used. You can tell that the software used by HGSS's primary composer was also used for Gen. 5 because if you assembled an unmodified Icirrus City .s file (this goes for some other songs, too) into a HGSS voicegroup, the song will actually sound correct (if you disregard the lower quality and differently pitched instruments used in HGSS)