I agree with Alan that Marx's analysis in Volume I of capital is not
limited to a special case where individual labor-contents of commodities
are well-defined and relative money prices are proportional to them, but
represents an abstraction applying to all commodity and capitalist
production.
"The commodity" of Volume I is best thought of as an average of all the
individual commodities, and the laws derived there as applying in this
average sense. I believe this point is clearly borne out by the
historical work that shows that Marx had written the notes we have as
Volume III before he polished Volume I for publication.
An implication of this is that the labor theory of value is not a theory
of price in Marx, but is consistent with any theory of the pricing of
individual commodities, including, for example, neoclassical equilibrium
theory. In my own view this is a decisive theoretical advantage, since a
theory that posits that individual commodities exchange in ratios
determined by their labor inputs is a) not applicable to cases of joint
production where it is not even possible to define the labor content of
individual commodities uniquely and b) clearly inconsistent with widely
accepted stylizations of the workings of a capitalist economy such as the
equalization of profit rates (which itself is only a particular theory of
competition and pricing).