Think of polyphonic as midi.  The sounds are fabricated by the phone
from a list of notes to play.  The Poly part is that there can be more
than one instrument playing at a time.  How many at a time depends on
how the music is written and the capabilities of the playback device.
Playback can sound very different on two different devices.

MP3 is a compressed digital recording of actual sound.

The V551 will do both.

H