[morzh]

First you want to test the transformer. You need to inject it with audio signal, say 1Vp-p, on the output side, and knowing the turns ratio (you do know it, don't you), say 100, measure the voltage at the input side (should be 100Vp-p). Using scope is the best, or if the frequency is low enough, a DMM will do. You have to measure the voltage right across the winding when injecting signal, and not at the generator's output.

Alternatively you could use a variac for the same purpose, BUT !!! inject the voltage in the INPUT side and measure the output at the output (speaker) side.
Use an isolation transformer before the variac lest you be zapped. It is not required for the experiment but is good for your safety. Use low voltage first, like 10-20V.