Dominic McIver Lopes, Hearing and Seeing Musical Expression, with Vincent Bergeron, Philosophy and Phenomenological Research 78 (2009) 1-16

Everybody assumes (1) that musical performances are sonic events and (2) that their expressive properties are sonic properties. This paper considers recent findings in the psychology of music perception that show that visual information combines with auditory information in the perception of musical expression. The findings show at the very least that arguments are needed for (1) and (2). If music expresses what we think it does, then its expressive properties may be visual as well as sonic; and if its expressive properties are purely sonic, then music expresses less than we think it does. And if the expressive properties of music are visual as well as sonic, then musical performances are not what we think they are - they are not purely sonic events.