Ohm's Law calculator
Indent your code.
There is no need to define functions for multiplication and division. It is just as clear to say a * b as mult(a,b).
getValueA and getValueB do exactly the same thing, except with a different global. The return value of these functions is ignored. It would be better to merge these into one and use the return value.
Don't use valA and valB for variable names. Use local variables with names like volt. 
Don't use a global for the result. Pass it as a parameter to displayResult, like this:

case 1:
    double amps = getValue("Amps");
    double ohms = getValue("Ohms");
    displayResult(amps * ohms, "volts");

You can, and should, avoid global variables completely here, as valueA and valueB are never used in the global context, they can be removed.
If the user types "no" or "N" it is treated as Yes. Handle this more carefully. For example, consider only the lowercase of the first letter.