Complex sentence: When Marge first speaks, she uses a muted trombone in parody of the "wah wah wah" voice that is used for adults in the "Peanuts" specials.
Split into simpler sentences: When Marge first speaks, she uses a muted trombone. This is a parody of the "wah wah wah" voice that is used for adults in the various "Peanuts" specials.