"The flowers that bloom in the spring, ___"
(Other definitions for tra-la that I've seen before include "Sound made when humming a song" , "Joyous refrain in song" , "Ironic; expression of joy or gaiety" , "Expression of joy or gaiety, musically" , "The flowers that bloom in the spring -- -- (Mikado)" .)
(Another definition for trala that I've seen is " Refrain from Gilbert and Sullivan".)