If I remember correctly I think Andre Watts also used to "vocalize" while
playing the piano. Sometimes it was loud enough for the audience to hear
it in the concert hall.  That might be disconcerting to some listeners or
just an affirmation that he was really into the piece and didn't even
realize he was doing it.  He still played very well, and it didn't seem
to turn people away from the box office.

Jane Pierce