Solution to Quodlibet

As suggested by the title of the puzzle and the sound file, the recording is of a new “Goldberg variation” each of whose eight phrases combines the corresponding phrase of one of Bach’s 30 variations (in the right hand = high range) with a popular tune (in the left hand = low range), slightly modified to make the harmony and transitions smoother. See the score here. For example, the beginning combines the start of the 3rd variation with “Happy Birthday To You”, and the end combines the end of the 11th variation with “Love Me Tender”. The eight letters of the answer PRACTICE are obtained by extracting from each tune’s name the n^th letter where n is the number of the variation played with it:

3 / haPpy birthday to you
8 / london bRidge is falling down
23 / my bonnie lies over the oceAn
13 / three blind miCe
5 / deck The halls
30 / what shall we do with the drunken saIlor [that's a capital I]
2 / sCarborough fair
11 / love me tendEr