There isn't by any chance a secondary battery needed for the sound unit? The analog boards in the sound freight cars use a 9v battery to keep the sound going when the track voltage is too low (below 9v, I guess). The sound gets pretty funky at low speed without the battery.
In my steam-sound-power-car I tried to eliminate the 9v battery by connecting to my NiMH pack but the sound board was not at all happy with that arrangement. It sounded a bit like what you described.
My Stainzes run pretty fast at 14v - too fast for 'reality', so I'd hope the sound unit would work at lower voltages.
"It is one of the happiest characteristics
of this glorious country that official utterances are invariably
regarded as unanswerable."
-Sir Joseph Porter, First Lord of the Admiralty (HMS Pinafore)