Yes, POLYPHONIC is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, L 1, Y 4, P 3, H 4, O 1, N 1, I 1, C 3.

Is it a valid word?

POLYPHONIC is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

having two or more phonetic values
of or relating to or characterized by polyphony
having two or more independent but harmonically related melodic parts sounding together