Question

///////////////////////////////////////
class A {
    ...
    const double funA(void)
    {...}
};

A a;
double x = a.funA(); 
// although the intention is to
// enforce the return value to be const and cannot be
// modified, it has little effect in the real world.

class A2 {
    ...
    double funB(void)
    {...}
};

///////////////////////////////////////
class A {
    void setA(const double d)
    { // now you cannot change the value of d, so what?
      // From my point of view, it is NOT a good practice to change the pass-in parameter
      // in this case, unless you want the caller to receive that change
      // instead, you can do 
      // const double dPassIn = d;
      / /then use dPassIn instead.
      ...
    }
};

class A2 {
    void setB(double d)
    {...}
};

//////////////////////////////////////

From my understanding, we should prefer to
using A2::funB and A2::setB because the const used in
both A::funA and A::setA has little meaning.

// Update //

    FMOD_RESULT F_API EventSystem::getReverbPresetByIndex(const int index, 
                                FMOD_REVERB_PROPERTIES *props, char **name = 0);

I consider FMOD is a well-designed package and it does use const int inside function parameter list.
Now, I agree that the A::setA(const double d) has its edge.

Answer 1

When returning by value the constant has no effect as it cannot be enforced anyway. Some compilers issue a warning. However it DOES make sense to return a pointer/reference to constant data.

When passing an argument into a function it is preferable (safer, allows for compiler optimizations) to pass it as a constant unless you absolutely need to change the data.