All the speaker does is move forwards and backwards. Its a linear motor.

If you take 2 signals (say 2 simple frequencies) and add them together, you get a new, combined signal.

Picture

The above combined signal is created from 2 simple sine waves with a high and a low frequency. If the speaker moves in the combined pattern, it will create waves of air pressure which will stimulate 2 different length hair cells in your cochlea.