You really don't need a AM modulator for sound. Slope detection works perfectly and sounds good. The only problem is that volume depends on RF level, and modern modulators have the aural carrier 15 db or more down compared to the visual carrier. In order to get decent volume you need to hit the set with at least +20 dBmv on the visual carrier.
We sell a modulator kit that is primarily for the British 405 system (AM video and sound), but it works for prewar American sets too.
http://www.earlytelevision.org/405_modulator.html