  Inhomogeneities in wave propagation conditions near and below the solar
surface have been detected by means of time-distance helioseismology. Here we
calculate the effect of temperature inhomogeneities on the travel times of
sound waves. A temperature increase, e.g. in active regions, not only increases
the sound speed but also lengthens the path along which the wave travels
because the expansion of the heated layers shifts the upper turning of the
waves upward. Using a ray tracing approximation we find that in many cases the
net effect of a temperature enhancement is an {\it increase} of the travel
times. We argue that the reduced travel times that are observed are caused by a
combination of magnetic fields in the active region and {\it reduced}
subsurface temperatures. Such a reduction may be related to the increased
radiative energy loss from small magnetic flux tubes.
