V? of coil when cone is tapped

[petemoore]

  Scenario: lightly but briskly tapping my finger on a full range 5''/8ohm speaker cone, what kind of ballpark average or peak voltage would the coil produce [driving a ~1meg input]?
  http://www.instructables.com/id/SPKR-MiK--How-to-make-a-microphone-from-a-speaker/
  He's made a drum microphone from a speaker..!
  He specified a 1:1 audio transformer, but it seems like the coil of the speaker would be DC free...and ...er...why add 2 more coils using the 1:1 ?