Anyone who has looked at the audio signal to a speaker on an oscilloscope can attest to the fact that it is a collection of frequencies and amplitudes.
A good way to go about looking at these signals would likely be to remove the speaker(s) and replace with a load resistor of 4, 8, or 16 ohms depending on amplifier output matching the impedance. Then on the line input to the audio amp inject a sine wave at a known frequency of 400 Hz, to 1 KHz to a few KHz. Increase the output level to the load (without toasting the load resistor) and observe the audio signal voltage on a good AC Voltmeter.
Ron