As j_garcia and Verdinut said, impedance varies with the frequency of the audio signal, which is an AC signal. A reading from a typical volt-ohm meter will give you a single resistance value, which makes sense only if you measure a DC signal. Any audio signal is AC.
Below is an example of a speaker that the manufacturer says is 'nominally 8 ohms'. Much of the range from 100 to 1,000 Hz is under 8 ohms. This is fairly typical of many 2-way loudspeakers.
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