what kind of voltage are speaker wires subject to?
You can calculate a reasonably good approximation of that based on the relation P = (E^2)/R. Power (in watts) = Voltage (in volts) squared divided by Resistance (in ohms).
So the maximum voltage would be the square root of the maximum power that would be fed to the speakers times their rated impedance.
That is an approximation because speaker impedance is not purely resistive, and is not constant with frequency, but it should be good enough for your purposes.
Also, however, power in this context is usually specified as rms rather than peak, so the calculated voltage would be rms also. If you want to know the worst case instantaneous voltage more precisely, to assess insulation requirements, multiply the worst case rms result by 1.414 (the square root of 2) to get the corresponding peak voltage.