The Sound of Music extended by and by (7)
I believe the answer is:
erelong
'by and by' is the definition.
'the sound of music extended' is the wordplay.
'the sound of' indicates a 'sounds like' (homophone) clue.
'music' becomes 'air' (air is a kind of music).
'extended' becomes 'long-' (similar in meaning).
'air' sounds like 'ere'.
'ere'+'long'='ERELONG'
(Other definitions for erelong that I've seen before include "Soon (poetically)" , "Soon (poetic)" , "Soon (arch.)" , "Soon (archaic)" .)