The item changes the background music to chiptune-like rearrangements of the tracks in the area that are similar to the original ones from Pokémon Gold, Silver, and Crystal. Sound effects and radio music are unaffected by the GB Sounds.
Despite the fact that the old-style tracks are supposed to sound like the original tracks from Pokémon Gold, Silver, and Crystal, none of them sound exactly like the originals. The acoustics sound different and many notes have other pitches. Some tracks had their tempos altered somewhat and other notes in various tracks are not even heard at all. This is due to the games using a soundfont for MIDI playback rather than generating real chiptunes. The different audio hardware between the Game Boy and the Nintendo DS would also have been taken into consideration.
If active during a Bug-Catching Contest, the GB Sounds will not switch to the old-style version of National Park during the contest results; however, it'll perform the switch after the results are announced.