Level 5-1 is a song from Super Noah's Ark 3D. The song is the same for both the DOS and SNES releases, but the different audio hardware plays them each with a distinct sound.
Super Noah's Ark 3D (SNES)
You will hear Level 5-1 song as soon as you start the first floor of the fifth level (see the screenshot), but overall you can hear it on Level 5-1 and Level 6-4.
The song is output to S-SMP chip.
Super Noah's Ark 3D (DOS)
In the DOS version, Level 5-1 plays in the same stages as in the Nintendo (E5M1 and E6M4).