Playing an instrument in KQ games.

[Sir Perceval of Daventry]

I've just noticed something:
It seems like playing an instrument at some point in a KQ game to solve a puzzle or get past an objective is almost like a part of the series.
The most notable examples I can think of:

Playing the Fiddle in KQ1
Playing the flute in KQ4 to charm and get past the Snake; Playing the Organ to open the Secret Room in Whaley Manor.
Playing the Harp in KQ5 to warm Queen Icebella's heart and distract the Harpies; Using the Tambourine to scare away the Snake and make Dink dance
Playing the Flute in KQ6 to distract the Garden plants and have them dance so as to get Hole in the Wall; Using the mechanical nightinggale to fool the guards of the Isle of Wonder; Using the nightinggale to gain Sing-Sing's trust.
Playing the Harp in KQ7 to gain access to the Fates' domain.

I'm not sure if there are other examples, but it seems almost like it's mandatory to play an instrument in a KQ game to get ahead. Each member of the Daventry Royal Family is musically talented in some way.

[Fierce Deity]

Yeah, it's typical with a lot of adventure games though, but I noticed it a lot in King's Quest. Music acts as an enchanting and soothing tool for the adventurer. It makes sense why you would need to play an instrument to tame a beast or reach an Ice Queen on an emotional level. Still, you see connections like that in all series. For instance, music plays a huge part in the Zelda series as well.

[Enchantermon]

You also have to play the lute in KQ4 and a flute in KQ7 to summon Count Tsepish's horse.

And, of course, the xylophone in KQ6's Isle of the Dead.

[Fierce Deity]

And, of course, the xylophone in KQ6's Isle of the Dead.

I love that part. 

[Baggins]

Mask of Eternity had the hearing horn, the bell, the gongs, and one of the jumping/block puzzles had a music element.