If your speakers are line powered, you should find line transformer.
Transformer which primary has line voltage and secondary is amplifier supply ac-voltage.
Secondary wires goes to rectifier diodes or bridge rectifier.
After rectifier , there is dc-voltage and this voltage is fed to filter capacitor.
You should find this capacitor, it has usually 470µF...4700µF capacitance marked on.
Voltage across this capacitor is amplifier supply voltage.
Be careful !!! Transformer primary has line voltage !!!
There is two ways to connect batteries
1.
to the wires from transformer secondary to diode bridge.
In this case diodes act as polarity protector for battery voltage-
Battery voltage = measured filter capacitor voltage + 1.5V for diodes
Transformer secondary wires must be disconnected.
2.
to the filter capacitor after diode bridge.
Battery voltage = measured filter capacitor voltage
Remember, in this poin voltage MUST have right polarity or you get damaged amplifier.
Transformer secondary wires must be disconnected.
Simple schematic.
View attachment 72133