The cone reproduces different frequencies by vibrating at different speeds. At low frequencies it moves in and out, say, 40 times a second. To reproduce high frequencies, it moves in and out at, say, 2,000 times a second. To reproduce a low frequency and a high frequency at the same time, it has to move in and out 40 times, while simultaneously moving in and out 2,000 times a second.
That's just to reproduce two frequencies. When a speaker is asked to reproduce a musical signal, it has to reproduce many different frequencies simultaneously.