IT is important for fire safety that the duplex be rated for the highest voltage it might be used with.
However the most important reason the makers will not comment on 220v use is legal.
They cannot afford to be sued just for saying "Sure go ahead and use the Duplex for 220V."
The blade design of the usual USA audio quality duplex are such that they can be used ONLY for 120Volts, with either a 15 amp circuit, or a 20 amp circuit. but NOT for 220 volts (in USA system)
My suggestion is to IGNORE the voltage issue in the duplex. but mainly consider the Amperage allowed. IF you calculate what wattage 20 amperes at 120 volts can be, vs your 220V at X amperage the point the amperage calculates to 'no greater' total wattage than the equivalent 120v at 20 amps.. If your use falls below that wattage (using 220v) then I would say you are safe to use them at 220v.
I would say keeping an eye on the new duplex for HEAT. if they heat up AT ALL (when used at your highest energy usage) they are not safe.
So good luck, and I hope they can work for you.
Be aware some folks will argue the points I wrote. Mainly for safety rules.
I like breaking rules....
Branson brings up a good point. Are you using a form of transformer to change your voltage for US type 120v equipment? If so then zero problem once the voltage is at 120v.
If you are using 220V and the equipment is adjusted to use 220V. Then what I wrote I still suggest. If the makers of the cords will not commit to how safe etc. Ask them just WHAT VOLTAGE IS THE WIRE INSULATION RATED TO? That is a question they certainly can answer.
Also a lot US powercord wire is good to 600 volts. Not all, but a lot if it.
In a wire, the voltage maximum IS an important specification. (more so in wire than in an outlet where the physical build of any outlet is usually way better at handling higher 220 volts))
Using wire with insulation which cannot handle the higher voltage is very dangerous.
