That's a 1/2 Watt speaker...so we're "only" talking maybe 1 Watt max for the two units.
Since this is well within the capability of the Voltage Converter, providing 10V AC is probably enough. That is, even if it was a 10V AC pure-sine, the voltage reaches ~14V peak which ought to be enough to maintain 9V DC output at light loads. Plus, if the 10V AC is from a chopped-sine controller fed by an 18V AC brick, the peaks are much higher.
I'd just try it. If you start to hear the tell-tale AC "hum" that will be the voltage converter output dipping from lack of enough peak input voltage. If it's just a little bit of hum you could turn down the Converter to, say, 8V DC. This will drop the volume a few dB but the voltage converter might be able to maintain a steady 8V so no hum. A simple tradeoff.
