Call a class method from inside the same class
You need to include roundto() in your class declaration, e.g. something like this:

class cSpeedOfSound {
public:
    cSpeedOfSound(float *i,float *C);

private:
    float roundto(float x, float dp);

    float *C;
    float *a;
    float *i;
};


In C++, class structures are fixed. You can define the body of a function almost anywhere, but all member functions/data must be specified in the original declaration.



As a side note, it's worth pointing out that you have a naming conflict in your code. You have member variables called i and C, and you have totally separate function parameters called i and C. The compiler won't complain about this (the function parameters will simply mask the member variables), but it may not do what you expect.