TrackedSymbols works well here as Jim points out. An alternative is to specify your function definitions in the Initialization option as in
Manipulate[
Plot[Evaluate[(f[x] + g[x]) /. {t -> (s/2), u -> 8*v}], {x, 0, 8*Pi}],
{s, 0, 20}, {v, 0, 6},
Initialization ->
{f[x_] := Sin[t*x],
g[x_] := Cos[u*x]}]
Note also that I put the replacement rule inside the Evaluate as this is more in line with what Evaluate's intended use is here.