Sorry I don't have FsX on this computer but thought I'd try. Does the ATC text and options come up when you hit the ATC button (on my PC
it's @ I think)? If not it could be the key has been reassigned which you could fix in controls assignments. If the text works but there's no speech I think it's under Options Setting ATC and then check "Use a pilots voice". At least in Fs9 it's like that. Last thing I can think of is under the sound settings maybe ATC has turned down to zero.
Sorry I don't have FsX with me now to give you better advice.