Manny Pacquiao's Aunt -- WE'RE A SINGING FAMILY ... Check Out These Pipes!

Call 'em the Filipino "Partridge Family" ... because it seems ALL of the Pacquiao's are obsessed with singing -- and Wednesday, Manny's Aunt Lilia flexed her pipes with a street rendition of "Let It Go!"

This video is amazing ... Manny's extremely spirited auntie is PUMPED for the fight -- and told us why she's convinced her nephew's gonna beat Floyd's ass!

Then, after more Pacquiao family members gathered around Auntie Lilia ... they broke out in song!