The Amazon theme plays when you're searching for the treasure in the Amazon jungle. The title comes from the DuckTales: Remastered album, which matches the in-game title of the stage.
This is the original version of the song. It is track 3 in the NSF file and played on the RP2A03.
The Game Boy release features the same music as the NES, just in slightly lower quality. The song is played on the DMG-CPU B.
DuckTales: Remastered (W32)
The Amazon was rearranged to pure digital music for the remastered game.