CDF Player and global function definitions

I am trying to embed interactive graphics in a CDF file

Manipulate[Plot[q*Sin[t] + (1 - q)*Sin[2*t], {t, 0, 2*Pi}], {q, 0, 1}]

works

ff[q_][t_] = q*Sin[t] + (1 - q)*Sin[2*t]
Manipulate[Plot[ff[q][t], {t, 0, 2*Pi}], {q, 0, 1}]

does not work

Module[{}, fff[q_][t_] = q*Sin[t] + (1 - q)*Sin[2*t];
   Manipulate[Plot[fff[q][t], {t, 0, 2*Pi}], {q, 0, 1}]]

does not work either

Is there a way to use function definitions for parametrized functions and then plot such functions with a slider for the parameter in CDF files?

There are two directions that you can go. If you wish to define your function ff outside of the Manipulate then you should use the option

SaveDefinitions->True

as in,

ff[q_, t_] := q*Sin[t] + (1 - q)*Sin[2*t]
Manipulate[Plot[ff[q, t], {t, 0, 2*Pi}], {q, 0, 1}, 
 SaveDefinitions -> True]

Note that I slightly changed your expression for ff to (a) have both variables with in a single argument structure (you could go either way) and (b) to make its definition a delayed evaluation (:=) rather than an immediate evaluation (=).

The alternative approach is to include the definition of ff in the Initialization option to Manipulate as in:

Manipulate[Plot[ff[q, t], {t, 0, 2*Pi}], {q, 0, 1}, 
 Initialization :> {ff[q_, t_] := q*Sin[t] + (1 - q)*Sin[2*t]}]

Note that the Initialization option is expressed as a delayed rule.