So we (Okada et al.) decided to try to nail this issue down with the following fMRI experiment: we localized auditory cortex in two different ways in individual subjects, using an AM noise localizer scan and using an anatomically defined mask covering Heschl's gyrus. We then measured the activity in the ROIs (left and right hemis) while listening to auditory speech only and while listening to auditory+visual speech. We found that the addition of the visual speech signal significantly up-regulated the activity of auditory cortex compared to auditory speech alone.
It's confirmed then: visual speech information drives auditory processing at the first stages of the cortical processing hierarchy.
Calvert, G. A., Bullmore, E. T., Brammer, M. J., Campbell, R., Williams, S. C. R., McGuire, P. K., Woodruff, P. W. R., Iversen, S. D., & David, A. S. (1997). Activation of auditory cortex during silent lipreading. Science, 276, 593-596.
(2013) An fMRI Study of Audiovisual Speech Perception Reveals Multisensory Interactions in Auditory Cortex. PLoS ONE 8(6): e68959. doi:10.1371/journal.pone.0068959
van Wassenhove, V., Grant, K. W., & Poeppel, D. (2005). Visual speech speeds up the neural processing of auditory speech. Procedings of the National Academy of Sciences, 102, 1181-1186.