Hi Mitchell,

I am glad you like the southern part of Germany! But unfortunately we do not have any snow yet! For this very reason I am right now in South Tyrol (Italy) to satisfy my addiction to XC-skating.

Regarding your question: To be honest - I cannot see that anything is not working correctly, everything shows and sums up as expected:

SetOptions[Plot, AspectRatio -> .3, GridLines -> Automatic, ImageSize -> Large];
Manipulate[
 GraphicsColumn[{Plot[Sin[(2 \[Pi])/\[Lambda] x - 2 \[Pi] f t], {x, 0, 10}, PlotRange -> {-1, 1}, 
    PlotLabel -> "Sin[(2 \[Pi])/\[Lambda] x-2 \[Pi] f t]"], 
   Plot[Sin[(2 \[Pi])/\[Lambda] x + 2 \[Pi] f t], {x, 0, 10}, PlotRange -> {-1, 1}, PlotLabel -> "Sin[(2 \[Pi])/\[Lambda] x+2 \[Pi] f t]"],
   Plot[Sin[(2 \[Pi])/\[Lambda] x - 2 \[Pi] f t] + Sin[(2 \[Pi])/\[Lambda] x + 2 \[Pi] f t], {x, 0, 10}, PlotRange -> {-2, 2}, 
    PlotLabel -> "Sin[(2 \[Pi])/\[Lambda] x-2 \[Pi] f t]+Sin[(2 \[Pi])/\[Lambda] x+2 \[Pi] f t]"]}], {t, 0, 3}, {\[Lambda], 1, 4}, {f, 1, 4}]

Regards -- Henrik

ADDENDUM:

I guess I now see your problem: When you move the frequency slider you see just a phase shift. But frequency has to do with time: depending your frequency adjustment if you move the time slider you will notice the difference.