First use the audioread function to import your .mp3 file into your workspace. (You may have to use the double function to convert it before you modulate it.) Then use the Signal Processing Toolbox modulate function, choosing the 'amdsb-tc' method to modulate it as an amplitude-modulated double sideband transmitted carrier signal. Then use the demod function with the same method to demodulate it. If you want to implement these functions yourself without using the built-in functions, the documentation goes into appropriate detail to describe the methods it uses to calculate them.
I’m not able to find similar blocks in Simulink other than QAM modulators in the Communications System Toolbox, and they aren’t what you want anyway. The SimRF Toolbox also doesn’t seem to have them.
