For those who don't know Monomane is a type of Japanese comedy show where comedians impersonate singers (among other acts) infront of an audience and usually other celebrities. Here's a vid of an Ayumi monomane (of the song Surreal) from a bit back.
http://www.youtube.com/watch?v=ibjaO5k0QEQ
I think she does a pretty good job not only capturing Ayu's voice but also her manneurisms!
