  We show that the squared speed of sound v_{s}^{2} is bounded from above at
high temperatures by the conformal value of 1/3 in a class of strongly coupled
four-dimensional field theories, given some mild technical assumptions. This
class consists of field theories that have gravity duals sourced by a single
scalar field. There are no known examples to date of field theories with
gravity duals for which v_{s}^{2} exceeds 1/3 in energetically favored
configurations. We conjecture that v_{s}^{2}=1/3 represents an upper bound for
a broad class of four-dimensional theories.
