Gone are the Pokémon Centers, Shops and Gyms. Gone are the houses, villagers and friendly Professors with angry nephews. It's just you and 5 generations of wild Pokémon. And you're pretty sure someone out there wants you very, very dead.
This music plays when you encounter most wild Pokémon in the grass. Other kinds of Pokémon encounters, such as legendaries, event encounters and "boss battles" in dungeons/temples have their own battle themes.
All music in P:SI is in MIDI format because I love the retro-ness of the format, and it doesn't sound out of place.