How remove noise from .wav file?

Hi, I need help, I have the signal :y(nT) =(x1(nT) + A1)cos(2pif1nT) +(x2(nT) + A2)cos(2pif2nT) where x1(nT) and x2(nT) are two real audio information signals with frequency band [10; 4000] Hz, f1 and f2 are the frequencies of the two sinusoidal carriers, where 4000 Hz <= f1 < f2 <= 19000 Hz (frequencies f1 and f2 are chosen such that there is no frequency overlap between the two modulated components, and specifically such that f2 - f1 >= 8500 Hz), and A1 and A2 are the amplitudes of the carriers. rite a MATLABr procedure that finds the two carrier frequencies f1, f2 and their amplitudes A1, A2 by means of a DFT analysis, extracts the two sinusoidal carriers by means of appropriate filtering with two band-pass filters with very narrow bandwidths, and demodulates the two audio information signals (multiplying the modulated signal by the previously extracted carriers and filtering the results with a bandpass filter having pass-band [10; 4000] Hz). The two resulting signals must be saved on the two channels of a stereo audio file in .wav format. In particular the procedure must:

1. read the file containing the input signal;
2. compute the spectrum of the input signal;
3. find the frequencies f1 and f2 of the sinusoidal carriers;
4. find an estimate of the amplitudes A1 and A2 of the carriers;
5. extracts the two carriers by filtering the input signal;
6. demodulate the two information signals;
7. design the filters so that the demodulated signals do not present audible distortions;
8. report the design parameters of the filters and plot their frequency resposnses;
9. compute the spectrum of the two demodulated signals.
10. write the file containing the two output signals.

I have arrived at step 4 but now I do not know how to proceed. who can help me understand how to proceed?

[edu ab]

place help i need the answers too