"Silence." said the bright commentator in a recent commercial short about canned music, "is becoming a very rare commodity." He was referring to swinging Britain, and it would seem that whether they take their music canned or live, the Poms are hooked on it like junkies. Intermingling with the ceaseless cacophony of London, this briskly edited kaleidoscope presents the singing Pom with all his native woodnotes, from 1812 by massed military bands to an old duck in an East End pub, from Beethoven in Croydon to the floral dance at Helston, from antique musical instruments to the radiophonic workshop of the BBC, from the Beatles to the last night of the Proms. It gets so that the guns of a royal salute are musical . . . and the quiet of the Cumberland Moors is bliss.