fourier series of unknown functions

Question

I am confused in understanding use of fourier expansions of functions. This answer, for example says that we can write voice as a sum of sines and cosines of different frequencies and amplitudes, but how can we find those coefficients without knowing what exactly function is ? Also, when we record a voice, do we really know the function (like $f(x) = x^2-2x$) ? And if we know the function, then what is the need to find its series expansion.
In many books, exercises contain questions like to expand a given function to its fourier series. If we already know a function (and that too a simple function, like f(x) = x), then why would I want to expand it into a series ?