The sampled audio data is the relative amplitude. If you would prefer a different kind of amplitude, you will need to specify. For example are you looking for the voltage on the LINE IN? Are you looking for what the microphone detected as pressure in Pascals?
MATLAB cannot represent continuous signals: continuous signals require infinite bandwidth.
You can, though, use MATLAB to calculate coefficients of a function that fits the known data at each time instant, with the function being continuous; the difficulty with that is that there are an infinite number of such functions (not just a "large number"), and there is no way to select which is the "right" function.