Trevor G: Sorry guys, but this looks like a massive con job.
1) IF the electronic shutter in an iPhone functioned in such a way that it would resolve the vibrations of a guitar string, it could only resolve one such string, because each string has a different frequency.
It might be able to resolve a particular harmonic frequency common to all strings, but I have serious doubts that that would be possible, too.
2) When the phone is placed inside the guitar the fingers, strings the bearded guy and the background are all out of focus - when the music starts the strings, the fingers and the background are all amazingly sharp. That's amazing DOF, if nothing else!
3) Check the light along the edges of the strings pre-music, with the way the light no longer "surrounds" the strings with-music.
4) The skin colour of the fingers is clearly visible when inserting the phone and switching it on, but they suddenly become stylised, sharp, black and colourless once the music is played.
Trevor G seems to belong to the same category of "skeptics" as those who don't believe NASA actually went to the moon.Another example of disbelief based on lack of knowledge and/or understanding would be if you thought that photo-finish photographs must be fakes, because the runners cannot have such weird arms or legs.