I can achieve the exact same result by going to System->Console Switches and enabling "Cart SDX", which would explain the ROM where the screen data should be. If you disable the aforementioned setting, all should be well, although output it not optimised for Altirra's POKEY emulation.
That was it! Thanks! Sounds good in Altirra with the following hacks: "e 44df d" "e 44e4 c".