As Neil said, you need a numerical value. The reason why: Sort sorts based on the function Order (or equivalently OrderedQ), which gives the canonical order of two expressions (not the numerical!).

Order[Sin[3], Sin[4]]
Order[N@Sin[3], N@Sin[4]]

The first gives 1 (meaning they are in order) the second gives -1 (meaning they are not in order)

This is because they Sin[3] and Sin[4] are left unevaluated in Mathematica. Written like that they are in order (3<4). So in order to have compare the numerical approximations of Sin[1] rather than "Sin[1]" you need to convert it using N (either before or in the pure function):

SortBy[{1, 2, 3, 4, 5, 6, 7, 8, 9}, N@*Sin]
SortBy[{1, 2, 3, 4, 5, 6, 7, 8, 9}, N@*Cos]
SortBy[{1, 2, 3, 4, 5, 6, 7, 8, 9}, N@*Minus@*Exp]

These problems will become easier in a future version of the Wolfram language as a new function will be introduced that I can't talk about yet... keep your eyes open for this functionality in future versions...