Who uses AF transformers these days ?? Curious to know.
Having *said* that, the idea of the transformer is of course to manage impedance. A typical magnetic speaker's coil is anywhere between 2 - 16 ohms usually. 4 & 8 ohms being the most common (at least at the time that i used to dabble in 'em some *years* ago).
However it's not the *voltage* that the transformer is managing - rather its the *power* - which stays the same, and is transferred to the load most efficiently when the impedances are suitably set.
Next - the word "impedance" implies a combination of resistive and reactive elements. Since the audio spectrum has such a large dynamic range, usually this impedance is specified for some standard fixed frequency. Probably 1KHz - but i'm guessing here.
The mechanics of a speaker add complications to the actual impedance. For a moving coil type of speaker, there are many many variables which affect this impedance over the audio frequency spectrum - hence its not just a simple "resistor-in-series-with-inductor" model.