There's an item named 'GB sounds' which stands for Game Boy sounds, and is only obtainable after you beat all 16 gym leaders. Despite the fact that the GB Sounds is supposed to sound like the original music from the Game Boy and Game Boy Color, none of the songs sound exactly like the originals. The reason that GB Sounds doesn't perfectly emulate the original soundtrack is due to the games using a sound font for MIDI playback, rather than generating real 8-bit chiptune sounds.