This short article suggests why when which bit of gear is not known to be
connected the subjective perceptions fail. In the brain integration of
information occurs using that from the ear and combining it with other
information to produce the final perception experience. When the bit of
gear is not known, but still in full sight, the subjective perceptions
disappear because the stored information about that gear and the sound
from it can't be integrated to form the subjective, and false, perception
experience.

http://www.eurekalert.org/pub_releas...-wtb101905.php

"Many activities would be difficult to carry out if the brain did not
receive information from a number of different sources at the same
time. Furthermore, by manipulating multisensory integration, one can
create illusions of perception. One well-known example is the
'ventriloquist effect'. If one hears a voice (for example, from a
loudspeaker), and then simultaneously sees a face or a mouth moving to
speak, then the voice appears to come from the mouth even when, in the
case of ventrioloquist, the mouth belongs to a dummy."